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Molecular beam epitaxial growth and characterization of ZnTe and CdTe on (001) GaAs
296
journal of Crystal Growth S6 )l98~)296—302 North—Holland, Anisterdarn
MOLECULAR BEAM EPITAXIAL GROWTH AND CHARACTERIZATION OF ZnTe AND CdTe ON (001) GaAs BK. WAGNER. J.D. OAKES and C.J. SUMMERS Microelectronics Research Center, Georgia Tech Research Institute, Atlanta, Georgia 30332. (ISA
Investigations of elemental diffusion in (001) and (UI) oriented CdTe and ZnTe layers grown by molecular beam epitaxy on (OW) GaAs substrates show high Ga diffusion along dislocations and defects generated at the substrate—epilayer interface. The use of CdTe/ZnTe superlattice buffer layers and nucleation on near-atomically planar GaAs surfaces is shown to suppress Ga diffusion to background detection levels.
1. Introduction There has been recent interest in epitaxial ZnTe, CdTe, HgTe, and their alloys because of their potential use in a wide variety of optoelectronic applications [1—4]. Zn ~Cd1 ~Te. for example. whose bandgap ranges from 1.5 to 2.3 eV has potential applications as a tunable source. detecbr. or solar cell in the visible region. Also. because their bandgaps are controllable by adjusting relative layer thicknesses, ZnTe/HgTe and ZnTe/CdTe strain layer superlattices (SLSs) have applications in the far-infrared and visible regions, respectively [5—71. Additionally. because they can be grown with low misfit defect generation. SLS’s have important structural applications such as acting as dislocation blocking layers for growing high quality Zn ~Cd1 ~Te, Hg1 ~Cd ~Te, and Hg1 — ~Zn ,,Te on different subtrates [7]. Because of the difficulty in obtaining high quality CdTe substrates or lattice matched substrates for the Hg1 .~~Cd~Te and Hg1 - ~Zn ,Te alloys much interest has been directed towards the growth of these materials by molecular beam epitaxy (MBE) on GaAs substrates for large area applications. In this paper we present a study of (001) and (111) oriented CdTe and ZnTe epitaxial layers grown on (001) GaAs substrates, and the use of secondary ion mass spectrometry (SIMS) to determine the extent of the interdiffusion between the epitaxial layer and the substrate. This study began with the deposition of CdTe
and ZnTe directly onto (001) GaAs and was then extended to investigate the Ga diffusion inhibiting effect of a surface oxide layer and a Zn ~Cd
1 ~Te buffer layer. Next, the combination of a buffer layer and a SLS to block the propogation of dislocations and defects was investigated. These studies and the observation of large irregularities in the Ga diffusion levels suggested that the quality of the original nucleation surface was an important factor affecting inter-diffusion across the substrate/epilayer interface. This hypothesis is supported by Sonnenfeld et al. who have demonstrated that standard GaAs cleaning procedures produce rough growth surfaces after oxide desorption which could provide defect nucleation sites for Ga diffusion channels into the epitaxial layers [8]. Therefore, (001) and (111) CdTe layers were also deposited on very planar MBE grown GaAs buffer layers. By growing a GaAs buffer layer it was possible to control the character of GaAs surface and terminate it with an As rich surface.
2. Experimental procedure Growth was performed in a Varian (len II MBE system equipped with a reflection high energy electron diffraction (RHEED) system. ZnTe and CdTe binary sources were used to deposit the epitaxial layers and were SN and 6N pure, respectively. Both the (001) and (111) oriented CdTe layers
0022-0248/88/$03.50 Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
BK. Wagner et aI.
/
MBE growth and characterization of Zn Te and CdTe on (001) GaAs
were grown on (001) GaAs substrates oriented 20 towards the (110) plane. The (001) CdTe layers were grown either by use of a thin oxide layer on the GaAs surface or by adsorbing a layer of Te on an oxide free GaAs surface before initiating CdTe growth. In the former method, when the RHEED pattern indicated that only a few monolayers of oxide remained on the GaAs surface at 580°C, the source shutter was opened and the substrate cooled to the growth temperature while the latter method involved the total removal of the oxide and cooling to the growth temperature under the CdTe flux. For the (111) CdTe layers, the substrate temperature was raised until the RHEED pattern indicated no surface oxide remained and then dropped to the growth temperature before growth was initiated. For both orientations, growth was carried out at a substrate temperature of 300°C and growth rate of 0.5 jim/h. This corresponded to a CdTe beam equivalent pressure (BEP) of 7 X iO~ Torr as measured by a nude ionization gauge mounted at the substrate position. The (001) ZnTe layers were grown on (001) GaAs substrates with and without ZnCdTe/CdTe buffer layers. In order to grow (111) ZnTe, it was necessary to first grow a (111) CdTe buffer layer. The (001) ZnTe layers deposited directly onto GaAs were either grown on an oxide layer analogous to the CdTe growth or were grown by entirely desorbing the oxide and allowing the substrate temperature to cool to the growth temperature under the ZnTe flux. To grow the (001) ZnCdTe buffer layers, 0.2 jim of CdTe was deposited before the ZnTe source shutter was opened and the substrate temperature raised over a ten minute period to the ZnTe growth temperature. After reaching the desired growth temperature, the CdTe shutter was closed and growth of the ZnTe proceeded. Growth rates from 0.39 to 0.54 jim/h were used corresponding to beam equivalent pressures of 7.2 X iO~ to 1.2 x 106 Torr, respectively. The (001) ZnTe layers were grown at ternperatures ranging from 350 to 420°C. For the (111) oriented ZnTe, however, RHEED studies indicated that epitaxy could only be initiated below 330°Con the CdTe buffer layer. At temperatures above 330°C the RHEED pattern was spotty —‘
297
even after nucleation and the growth of 500 A of material. Thus growth temperatures from 270 to 330°Cwere used. For (001) ZnTe deposited directly on GaAs growth occured with a (2 x 1) Te stabilized surface analogous to (001) CdTe growth [4]. However, eventually the (2 x 1) pattern changed to a (6 X 1) and finally a (3 x 1) reconstruction. It was ohserved that the change to the (6 x 1) pattern occurred at a ZnTe thickness of 1.5 um. The layers grown on (001) CdTe/ZnCdTe buffers, however, typically did not become (6 X 1) reconstructed until the ZnTe thickness exceeded 2 jim. The observed (6 x 1) pattern was not complete in that only the three inner fractional order streaks were visible. The spacing of the streaks, however, mdicated that the azimuth was six-fold reconstructed. The GaAs layers were grown in a Varian Gen-Il MBE dedicated to the growth of Ill—V materials. After 0.5 jim of growth, RHEED indicated that the MBE GaAs layers were extremely smooth and exhibited a (2 X 4) As stabilized surface during growth. The substrate was then cooled to 15°C and an amorphous As layer deposited to passivate the surface. The substrates were quickly transferred to the Il-VI MBE equipment in a nitrogen glove bag and the amorphous As layer desorbed at 360°C leaving a smooth As terminated highly crystalline growth surface. While raising the substrate temperature, the GaAs RHEED pattern was monitored to ensure that the surface did not become Ga rich. Secondary ion mass spectroscopy (SIMS) analysis was performed in an Atomica system to determine the extent of elemental interdiffusion across the interface. SIMS depth profiles and line scans employed a 150 nA argon ion beam with scan widths from 0.36 to 2 mm.
3. Results and discussion Fig. la shows the depth profile for an (001) ZnTe layer grown directly on GaAs. As shown, Ga diffuses rapidly from the substrate to the ZnTe surface. Diffusion of Zn and Te into the GaAs is also seen. Similar results were observed for (111) ZnTe layers grown on CdTe buffer layers as shown
298
B. K. Wagner et al.
/ MBE growth and characterization
by fig. lb. However, it is apparent that both the (111) ZnTe/(111) CdTe and the (111) CdTe/(001)
________________________________
a
GaAs interfaces are sharper than those obtained for (001) growth. Closer inspection of the data and profiles taken on different regions of the sample
6 GA
showed gross inhomogeneities in both the Ga depth profiles and the concentration levels. For this reason the imaging mode of the SIMS was used in conjunction with depth profiling to investigate both the lateral and depth distribution of Ga. The
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Fig. 1. Elemental profiles measured by SIMS In: (a) a (001) ZnTe layer grown by MBE on a (001) GaAs substrate; (b) a (111) ZnTe layer grown by MBE on a (001) GaAs substrate.
II Fig. 2. SIMS image of the Ga distribution .icro~s a 1 mm square in MBE grown layers: (a) (001) ZnTe on (0111) (,aAs~ (b) (111) ZnTe on (001) GaAs.
BK. Wagner et al.
/ MBE growth and characterization of Zn Te and CdTe on
and (111) ZnTe are shown in fig. 2. As seen, there are local areas of high Ga concentration or “hot spots” in these layers. SIMS analysis of the (001) and (111) oriented CdTe layers also indicated that for both orientations Ga readily diffuses to the surface of the layers. However, the Ga count in the (111) layer was smaller than the relatively high level exhibited in the (111) CdTe buffer of Figure lb. Also, lateral images of these layers displayed a more uniform Ga distribution for the (001) and (111) CdTe layers without the large “hot spots” that are characteristic of the ZnTe layers. This is indicative of the higher quality of the CdTe layers as compared to the ZnTe layers. This is surprising since the lattice mismatch between CdTe and GaAs is larger than the mismatch between ZnTe and GaAs. Due to the large lattice mismatch in both material systems it was hypothesized that the Ga “hot spots” are the result of enhanced diffusion along defect networks propogating from the substrate epilayer interface. In order to further quantify the nature of the “hot spots,” SIMS line scans were undertaken in areas exhibiting high Ga concentrations. These experiments allowed a relative comparison of the background Ga level in the layers with the Ga level in the “hot spots.” At present this is only a relative scaling due to matrix effects which have been neglected. In the case of CdTe/GaAs structures, both the (001) and (111) orientations were compared. Fig. 3 shows the Ga lateral image and line scan of (111) CdTe which indicates an order of magnitude difference between the background and “hot spot” Ga levels. The (001) CdTe layers showed approximately the same Ga concentration levels. In neither case were any As counts noted in the “hot spots” nor was a defficiency of Cd or Te apparent. Fig. 4 shows Ga imaging and line scans of (001) ZnTe grown with the oxide left on the GaAs, without an oxide, and with a CdTe/ZnCdTe buffer layer. In the case of the ZnTe grown on the oxide layer (fig. 4a), the linescan indicated that the “hot spot” had a Ga concentration which was two orders of magnitude greater than the background level. Also, within the Ga “hot spot,” both the Zn and Te counts were observed to increase by a factor of 1.5. The ZnTe grown without the surface
(001) GaAs
299
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Fig. 3. (a) SIMS image of Ga distribution in (1111 C dTe on (001) GaAs; (b) linescan showing Ga intensity through center of image.
oxide (fig. 4b) has fewer and less intense “hot spots” but the uniform background Ga count was 15 times greater than measured for ZnTe grown on a thin oxide layer. Thus the oxide may help to inhibit Ga diffusion because the Ga is more removed from the initial growth surface. However, the possibility of nonuniform oxide coverage may result in increased numbers of dislocations and defects. Finally, as shown by fig. 4c, the Ga concentration of the (001) ZnTe layers grown with the CdTe/ZnCdTe buffer layer was slightly less than that observed for ZnTe grown on the oxide layer. Because of the ability of strained layer superlattices to act as dislocation filters, the effect of
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C Fig. 4. SIMS image of the Ga distribution in WOl Zn’Ie lasers grossn bs NIH! on (001) ( ~a.\s substrates: Ia ssith oside left on surfaLe: (hI ~ ithout an oxide: )c( n iili a CdTe /.n( d Ic huller a’. cr
B.K. Wagner et aL
/ MBE growth and characterization ofZnTe and CdTe on (001)
GaAs
301
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DEPTH [MICRONS] Fig. 5. SIMS image of Ga distribution in (001) CdTe grown on a MBE grown (001) GaAs buffer layer: (a) 200 A from interface; (h) 0.5 pm from interface; (c) 1.0 pm from interface.
Fig. 6. Elemental profiles measured for CdTe grown on MBE grown (001) GaAs buffer layers: (a)(001) orientation; (h)(l11) orientation.
302
BK. Wagner et al.
/
.~4BEgrowth and characterization ofZnTe and C~dTeon (00/) GaA
CdTe/ZnTe SLSs to eliminate the Ga “hot spots” was investigated. For these structures, the built was tailored by using various CdTe/ZnTe thickness ratios. Monfroy et a]. have reported that a Zn5Cd1 ~Te buffer layer with x = 0.5 served as the best growth surface for the CdTe/ZnTe SLSs [7]. Thus, 0.2 jim of Zn0 5Cd0’.Te was grown before initiating the SLS growth. All of the SLSs were grown at 300°C with a 0.5 jim/h growth rate and using a constant SL period and total thickness of 50 and 2500 A, respectively, ZnTe/CdTe thickness ratios of 25/25, 20/30, and 15/35 were investigated. After the SLSs were deposited, approximately 2 jim of CdTe was grown. Analysis of the Ga profile in these structures indicated that the binary SLSs were very effective and that an order of magnitude reduction in the Ga concentration was obtained over that possible with the ZnCdTe/CdTe buffer. However, no appreciable variation was observed between the different SL structures. Recent work by Gunshore has demonstrated the advantage of the initiating growth of lI—VI compounds on a smooth high quality GaAs surface [9]. To determine the effect of such a nucleation surface (001) and (111) CdTe layers were deposited on MBE grown GaAs buffer layers. Fig. 5 shows lateral Ga images at progressively larger thicknesses in a (001) CdTe layer deposited on a GaAs buffer layer. It is evident that the Ga concentration in the layer has been reduced to very low levels at a thickness of 1 jim. These results are confirmed by fig. 6. which shows depth profiles for (001) and (111) CdTe grown on GaAs buffer layers. The greater Ga rejection in the (111) CdTe as opposed to the (001) CdTe layer is attributed to the higher quality of (111) CdTe as indicated by X-ray double crystal rocking curves. -
.
Characterization by SIMS depth profiles and lateral imaging has shown the existence of Ga “hot spots” in these layers which are believed to he due to enhanced Ga diffusion along defects propagating from the substrate/epilayer interface. To inhibit the Ga diffusion process, several blocking techniques were investigated. These consisted of oxide layers, ZnCdTe/CdTe buffer layers. CdTe/ZnTe superlattice buffer layers, and MBE GaAs buffer layers. The first three methods were somewhat successful with the superlattice structure producing up to two orders of magnitude reduction in the Ga concentration; however, the MBE GaAs buffer proved to be the most effective in reducing Ga diffusion. It is believed that improved substrate preparation techniques combined with SLS buffer layers will eliminate the need for the MBE grown GaAs buffer layers and enable high quality and high purity CdTe and ZnTe layers to be grown on GaAs.
References [1] RD. Feldman. R.F. Austin. A.H. Dagem and El-I. Westcrick, AppI. Phys. Letters 49(1986)797. [2] J.H. Dinan and SB. Qadri, Thin Solid Films 131 (1985)
267
[31J.H.
Dinan and S.B. Qadri, J. Vacuum Sci. Technol. A3
(1985) 851.
[41J.D.
Benson, BK. Wagner, A. Torabi and Ci. Summers.
AppI. Phys. Letters 49 (1986) 1034. [5] J.P. Faurie, S. Sivananthan. X. Chu and P.A. Wyewarnasuriya, App] Phys. Letters 48 (1986) 785. [6] R.H. Miles. G.Y. Wu, MB. Johnson. T.C. McGill, J.P. Faurie and S. Sivananthan. AppI. Phys. Letters 48 (1986) 1383. [7] G. Monfroy. S. Sivananthan, X. Chu, J.P. Faurie. R.D. Knox and J.L. Staudenmann. AppI. Ph~s.Letters 49 (1986) 152 [8] R. Sonnenfeld, J. Schneir, B. Drake. P.K. Hansma and
D.E. Aspnes, AppI. Phys. Letters 50 (1987) 1742. L.A. Koledziejski, MR. Mellock, M. Vaziri,
[91R.L. Gunshor,
4.
Conclusion
Epitaxial growth of (001) and (111) ZnTe and CdTe layers on (001) GaAs has been carried out.
C. Choi and N. Otsuka. AppI. Phys. Letters 50 (1987) 200.
journal of Crystal Growth S6 )l98~)296—302 North—Holland, Anisterdarn
MOLECULAR BEAM EPITAXIAL GROWTH AND CHARACTERIZATION OF ZnTe AND CdTe ON (001) GaAs BK. WAGNER. J.D. OAKES and C.J. SUMMERS Microelectronics Research Center, Georgia Tech Research Institute, Atlanta, Georgia 30332. (ISA
Investigations of elemental diffusion in (001) and (UI) oriented CdTe and ZnTe layers grown by molecular beam epitaxy on (OW) GaAs substrates show high Ga diffusion along dislocations and defects generated at the substrate—epilayer interface. The use of CdTe/ZnTe superlattice buffer layers and nucleation on near-atomically planar GaAs surfaces is shown to suppress Ga diffusion to background detection levels.
1. Introduction There has been recent interest in epitaxial ZnTe, CdTe, HgTe, and their alloys because of their potential use in a wide variety of optoelectronic applications [1—4]. Zn ~Cd1 ~Te. for example. whose bandgap ranges from 1.5 to 2.3 eV has potential applications as a tunable source. detecbr. or solar cell in the visible region. Also. because their bandgaps are controllable by adjusting relative layer thicknesses, ZnTe/HgTe and ZnTe/CdTe strain layer superlattices (SLSs) have applications in the far-infrared and visible regions, respectively [5—71. Additionally. because they can be grown with low misfit defect generation. SLS’s have important structural applications such as acting as dislocation blocking layers for growing high quality Zn ~Cd1 ~Te, Hg1 ~Cd ~Te, and Hg1 — ~Zn ,,Te on different subtrates [7]. Because of the difficulty in obtaining high quality CdTe substrates or lattice matched substrates for the Hg1 .~~Cd~Te and Hg1 - ~Zn ,Te alloys much interest has been directed towards the growth of these materials by molecular beam epitaxy (MBE) on GaAs substrates for large area applications. In this paper we present a study of (001) and (111) oriented CdTe and ZnTe epitaxial layers grown on (001) GaAs substrates, and the use of secondary ion mass spectrometry (SIMS) to determine the extent of the interdiffusion between the epitaxial layer and the substrate. This study began with the deposition of CdTe
and ZnTe directly onto (001) GaAs and was then extended to investigate the Ga diffusion inhibiting effect of a surface oxide layer and a Zn ~Cd
1 ~Te buffer layer. Next, the combination of a buffer layer and a SLS to block the propogation of dislocations and defects was investigated. These studies and the observation of large irregularities in the Ga diffusion levels suggested that the quality of the original nucleation surface was an important factor affecting inter-diffusion across the substrate/epilayer interface. This hypothesis is supported by Sonnenfeld et al. who have demonstrated that standard GaAs cleaning procedures produce rough growth surfaces after oxide desorption which could provide defect nucleation sites for Ga diffusion channels into the epitaxial layers [8]. Therefore, (001) and (111) CdTe layers were also deposited on very planar MBE grown GaAs buffer layers. By growing a GaAs buffer layer it was possible to control the character of GaAs surface and terminate it with an As rich surface.
2. Experimental procedure Growth was performed in a Varian (len II MBE system equipped with a reflection high energy electron diffraction (RHEED) system. ZnTe and CdTe binary sources were used to deposit the epitaxial layers and were SN and 6N pure, respectively. Both the (001) and (111) oriented CdTe layers
0022-0248/88/$03.50 Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
BK. Wagner et aI.
/
MBE growth and characterization of Zn Te and CdTe on (001) GaAs
were grown on (001) GaAs substrates oriented 20 towards the (110) plane. The (001) CdTe layers were grown either by use of a thin oxide layer on the GaAs surface or by adsorbing a layer of Te on an oxide free GaAs surface before initiating CdTe growth. In the former method, when the RHEED pattern indicated that only a few monolayers of oxide remained on the GaAs surface at 580°C, the source shutter was opened and the substrate cooled to the growth temperature while the latter method involved the total removal of the oxide and cooling to the growth temperature under the CdTe flux. For the (111) CdTe layers, the substrate temperature was raised until the RHEED pattern indicated no surface oxide remained and then dropped to the growth temperature before growth was initiated. For both orientations, growth was carried out at a substrate temperature of 300°C and growth rate of 0.5 jim/h. This corresponded to a CdTe beam equivalent pressure (BEP) of 7 X iO~ Torr as measured by a nude ionization gauge mounted at the substrate position. The (001) ZnTe layers were grown on (001) GaAs substrates with and without ZnCdTe/CdTe buffer layers. In order to grow (111) ZnTe, it was necessary to first grow a (111) CdTe buffer layer. The (001) ZnTe layers deposited directly onto GaAs were either grown on an oxide layer analogous to the CdTe growth or were grown by entirely desorbing the oxide and allowing the substrate temperature to cool to the growth temperature under the ZnTe flux. To grow the (001) ZnCdTe buffer layers, 0.2 jim of CdTe was deposited before the ZnTe source shutter was opened and the substrate temperature raised over a ten minute period to the ZnTe growth temperature. After reaching the desired growth temperature, the CdTe shutter was closed and growth of the ZnTe proceeded. Growth rates from 0.39 to 0.54 jim/h were used corresponding to beam equivalent pressures of 7.2 X iO~ to 1.2 x 106 Torr, respectively. The (001) ZnTe layers were grown at ternperatures ranging from 350 to 420°C. For the (111) oriented ZnTe, however, RHEED studies indicated that epitaxy could only be initiated below 330°Con the CdTe buffer layer. At temperatures above 330°C the RHEED pattern was spotty —‘
297
even after nucleation and the growth of 500 A of material. Thus growth temperatures from 270 to 330°Cwere used. For (001) ZnTe deposited directly on GaAs growth occured with a (2 x 1) Te stabilized surface analogous to (001) CdTe growth [4]. However, eventually the (2 x 1) pattern changed to a (6 X 1) and finally a (3 x 1) reconstruction. It was ohserved that the change to the (6 x 1) pattern occurred at a ZnTe thickness of 1.5 um. The layers grown on (001) CdTe/ZnCdTe buffers, however, typically did not become (6 X 1) reconstructed until the ZnTe thickness exceeded 2 jim. The observed (6 x 1) pattern was not complete in that only the three inner fractional order streaks were visible. The spacing of the streaks, however, mdicated that the azimuth was six-fold reconstructed. The GaAs layers were grown in a Varian Gen-Il MBE dedicated to the growth of Ill—V materials. After 0.5 jim of growth, RHEED indicated that the MBE GaAs layers were extremely smooth and exhibited a (2 X 4) As stabilized surface during growth. The substrate was then cooled to 15°C and an amorphous As layer deposited to passivate the surface. The substrates were quickly transferred to the Il-VI MBE equipment in a nitrogen glove bag and the amorphous As layer desorbed at 360°C leaving a smooth As terminated highly crystalline growth surface. While raising the substrate temperature, the GaAs RHEED pattern was monitored to ensure that the surface did not become Ga rich. Secondary ion mass spectroscopy (SIMS) analysis was performed in an Atomica system to determine the extent of elemental interdiffusion across the interface. SIMS depth profiles and line scans employed a 150 nA argon ion beam with scan widths from 0.36 to 2 mm.
3. Results and discussion Fig. la shows the depth profile for an (001) ZnTe layer grown directly on GaAs. As shown, Ga diffuses rapidly from the substrate to the ZnTe surface. Diffusion of Zn and Te into the GaAs is also seen. Similar results were observed for (111) ZnTe layers grown on CdTe buffer layers as shown
298
B. K. Wagner et al.
/ MBE growth and characterization
by fig. lb. However, it is apparent that both the (111) ZnTe/(111) CdTe and the (111) CdTe/(001)
________________________________
a
GaAs interfaces are sharper than those obtained for (001) growth. Closer inspection of the data and profiles taken on different regions of the sample
6 GA
showed gross inhomogeneities in both the Ga depth profiles and the concentration levels. For this reason the imaging mode of the SIMS was used in conjunction with depth profiling to investigate both the lateral and depth distribution of Ga. The
10~
)
~ 1.21..
ofZn Te and CdTe on (001) GaAs
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results of a lateral imaging study of Ga for (001)
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Fig. 1. Elemental profiles measured by SIMS In: (a) a (001) ZnTe layer grown by MBE on a (001) GaAs substrate; (b) a (111) ZnTe layer grown by MBE on a (001) GaAs substrate.
II Fig. 2. SIMS image of the Ga distribution .icro~s a 1 mm square in MBE grown layers: (a) (001) ZnTe on (0111) (,aAs~ (b) (111) ZnTe on (001) GaAs.
BK. Wagner et al.
/ MBE growth and characterization of Zn Te and CdTe on
and (111) ZnTe are shown in fig. 2. As seen, there are local areas of high Ga concentration or “hot spots” in these layers. SIMS analysis of the (001) and (111) oriented CdTe layers also indicated that for both orientations Ga readily diffuses to the surface of the layers. However, the Ga count in the (111) layer was smaller than the relatively high level exhibited in the (111) CdTe buffer of Figure lb. Also, lateral images of these layers displayed a more uniform Ga distribution for the (001) and (111) CdTe layers without the large “hot spots” that are characteristic of the ZnTe layers. This is indicative of the higher quality of the CdTe layers as compared to the ZnTe layers. This is surprising since the lattice mismatch between CdTe and GaAs is larger than the mismatch between ZnTe and GaAs. Due to the large lattice mismatch in both material systems it was hypothesized that the Ga “hot spots” are the result of enhanced diffusion along defect networks propogating from the substrate epilayer interface. In order to further quantify the nature of the “hot spots,” SIMS line scans were undertaken in areas exhibiting high Ga concentrations. These experiments allowed a relative comparison of the background Ga level in the layers with the Ga level in the “hot spots.” At present this is only a relative scaling due to matrix effects which have been neglected. In the case of CdTe/GaAs structures, both the (001) and (111) orientations were compared. Fig. 3 shows the Ga lateral image and line scan of (111) CdTe which indicates an order of magnitude difference between the background and “hot spot” Ga levels. The (001) CdTe layers showed approximately the same Ga concentration levels. In neither case were any As counts noted in the “hot spots” nor was a defficiency of Cd or Te apparent. Fig. 4 shows Ga imaging and line scans of (001) ZnTe grown with the oxide left on the GaAs, without an oxide, and with a CdTe/ZnCdTe buffer layer. In the case of the ZnTe grown on the oxide layer (fig. 4a), the linescan indicated that the “hot spot” had a Ga concentration which was two orders of magnitude greater than the background level. Also, within the Ga “hot spot,” both the Zn and Te counts were observed to increase by a factor of 1.5. The ZnTe grown without the surface
(001) GaAs
299
_____ _______________
_____________________________________________ __________
_____
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Fig. 3. (a) SIMS image of Ga distribution in (1111 C dTe on (001) GaAs; (b) linescan showing Ga intensity through center of image.
oxide (fig. 4b) has fewer and less intense “hot spots” but the uniform background Ga count was 15 times greater than measured for ZnTe grown on a thin oxide layer. Thus the oxide may help to inhibit Ga diffusion because the Ga is more removed from the initial growth surface. However, the possibility of nonuniform oxide coverage may result in increased numbers of dislocations and defects. Finally, as shown by fig. 4c, the Ga concentration of the (001) ZnTe layers grown with the CdTe/ZnCdTe buffer layer was slightly less than that observed for ZnTe grown on the oxide layer. Because of the ability of strained layer superlattices to act as dislocation filters, the effect of
95)
It K. U agner r cii
.5-.
tIRE groG tic cipid /iar~u~ 1/cation ccl /n I ~jnj ( j/
oi~((till) ( ,a I
____
a
______________________
________
b
,
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-
C Fig. 4. SIMS image of the Ga distribution in WOl Zn’Ie lasers grossn bs NIH! on (001) ( ~a.\s substrates: Ia ssith oside left on surfaLe: (hI ~ ithout an oxide: )c( n iili a CdTe /.n( d Ic huller a’. cr
B.K. Wagner et aL
/ MBE growth and characterization ofZnTe and CdTe on (001)
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DEPTH [MICRONS] Fig. 5. SIMS image of Ga distribution in (001) CdTe grown on a MBE grown (001) GaAs buffer layer: (a) 200 A from interface; (h) 0.5 pm from interface; (c) 1.0 pm from interface.
Fig. 6. Elemental profiles measured for CdTe grown on MBE grown (001) GaAs buffer layers: (a)(001) orientation; (h)(l11) orientation.
302
BK. Wagner et al.
/
.~4BEgrowth and characterization ofZnTe and C~dTeon (00/) GaA
CdTe/ZnTe SLSs to eliminate the Ga “hot spots” was investigated. For these structures, the built was tailored by using various CdTe/ZnTe thickness ratios. Monfroy et a]. have reported that a Zn5Cd1 ~Te buffer layer with x = 0.5 served as the best growth surface for the CdTe/ZnTe SLSs [7]. Thus, 0.2 jim of Zn0 5Cd0’.Te was grown before initiating the SLS growth. All of the SLSs were grown at 300°C with a 0.5 jim/h growth rate and using a constant SL period and total thickness of 50 and 2500 A, respectively, ZnTe/CdTe thickness ratios of 25/25, 20/30, and 15/35 were investigated. After the SLSs were deposited, approximately 2 jim of CdTe was grown. Analysis of the Ga profile in these structures indicated that the binary SLSs were very effective and that an order of magnitude reduction in the Ga concentration was obtained over that possible with the ZnCdTe/CdTe buffer. However, no appreciable variation was observed between the different SL structures. Recent work by Gunshore has demonstrated the advantage of the initiating growth of lI—VI compounds on a smooth high quality GaAs surface [9]. To determine the effect of such a nucleation surface (001) and (111) CdTe layers were deposited on MBE grown GaAs buffer layers. Fig. 5 shows lateral Ga images at progressively larger thicknesses in a (001) CdTe layer deposited on a GaAs buffer layer. It is evident that the Ga concentration in the layer has been reduced to very low levels at a thickness of 1 jim. These results are confirmed by fig. 6. which shows depth profiles for (001) and (111) CdTe grown on GaAs buffer layers. The greater Ga rejection in the (111) CdTe as opposed to the (001) CdTe layer is attributed to the higher quality of (111) CdTe as indicated by X-ray double crystal rocking curves. -
.
Characterization by SIMS depth profiles and lateral imaging has shown the existence of Ga “hot spots” in these layers which are believed to he due to enhanced Ga diffusion along defects propagating from the substrate/epilayer interface. To inhibit the Ga diffusion process, several blocking techniques were investigated. These consisted of oxide layers, ZnCdTe/CdTe buffer layers. CdTe/ZnTe superlattice buffer layers, and MBE GaAs buffer layers. The first three methods were somewhat successful with the superlattice structure producing up to two orders of magnitude reduction in the Ga concentration; however, the MBE GaAs buffer proved to be the most effective in reducing Ga diffusion. It is believed that improved substrate preparation techniques combined with SLS buffer layers will eliminate the need for the MBE grown GaAs buffer layers and enable high quality and high purity CdTe and ZnTe layers to be grown on GaAs.
References [1] RD. Feldman. R.F. Austin. A.H. Dagem and El-I. Westcrick, AppI. Phys. Letters 49(1986)797. [2] J.H. Dinan and SB. Qadri, Thin Solid Films 131 (1985)
267
[31J.H.
Dinan and S.B. Qadri, J. Vacuum Sci. Technol. A3
(1985) 851.
[41J.D.
Benson, BK. Wagner, A. Torabi and Ci. Summers.
AppI. Phys. Letters 49 (1986) 1034. [5] J.P. Faurie, S. Sivananthan. X. Chu and P.A. Wyewarnasuriya, App] Phys. Letters 48 (1986) 785. [6] R.H. Miles. G.Y. Wu, MB. Johnson. T.C. McGill, J.P. Faurie and S. Sivananthan. AppI. Phys. Letters 48 (1986) 1383. [7] G. Monfroy. S. Sivananthan, X. Chu, J.P. Faurie. R.D. Knox and J.L. Staudenmann. AppI. Ph~s.Letters 49 (1986) 152 [8] R. Sonnenfeld, J. Schneir, B. Drake. P.K. Hansma and
D.E. Aspnes, AppI. Phys. Letters 50 (1987) 1742. L.A. Koledziejski, MR. Mellock, M. Vaziri,
[91R.L. Gunshor,
4.
Conclusion
Epitaxial growth of (001) and (111) ZnTe and CdTe layers on (001) GaAs has been carried out.
C. Choi and N. Otsuka. AppI. Phys. Letters 50 (1987) 200.