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AlAs based resonant tunneling diodes grown on patterned and non-patterned GaAs(100) substrates
1110
Journal of Crystal Growth 111 (1991) 1110—1115 North-Holland
Highly strained pseudomorphic In~Ga1_~As/AIAs based resonant tunneling diodes grown on patterned and non-patterned GaAs( 100) substrates R.M. Kapre, A. Madhukar and S. Guha Photonic Materials and Devices Laboratory, University of Southern California; Los Angeles, California 90089-0241, USA
Resonant tumieling diodes (RTDs) with strained well and spacer regions made of In~Ga1_~Asalloys or (InAs)M/(GaAs)N short period multiple quantum well (SPMQW) have been fabricated on GaAs(100) substrates. With increasing x, the presence of strain results in generation of misfit dislocations and/or change to three-dimensional island mode of growth, both detrimental to the performance of the RTDs. Consequently, a systematic study of the electrical and structural properties of RTDs with 0.10 x 0.33 was carried out with emphasis on control of the growth kinetics. This has led to RTDs with (InAs)1/(GaAs)2 room 2. This is the firstwells timehaving that devices grown on GaAs any thick strain relieving intermediate layers shown simultaneously high PVRs and J~,. temperature peaksubstrate to valleywithout ratios (PVR) of 4.7 with peak current densities (Jr)have of 125 kA/cm
1. Introduction Resonant tunneling diodes (RTDs) have been fabricated in various material systems including GaAs/A1GaAs, In 0 53Ga047As/ALks, and InAs/ A1Sb grown on GaAs, InP, and InAs substrates, respectively. Considering the usual figures of merit of peak current density (Jr) and peak-to-valley ratio (PVR), strained RTDs with In0 53Ga047As/ InAs wells and AlAs barriers grown on InP substrate have shown the best results: a room 2 [1]temperand a ature PVR of 30 withJ~, 400—500 6 kA/cmkA/cm2 [2]. PVR of 3—4 with Lattice matched GaAs/ A1GaAs RTDs on =
=
GaAs(100) substrates have shown room tempera2 [3] ture PVR PVRsofof 4—5with with J~,100—200 10—20 kA/cm2 kA/cm [4]. and 2—2.5 Strained InGaAs/AIAs RTDs on GaAs(100) have shown comparable behaviour with PVR of 4.7 and 10—12 kA/cm2 [5].InAs/A1Sb devices based on thick n~-InAs epilayers on top of GaAs (i.e. very low strain in the RTD) have yielded room temperature J~, 200—400 kA/cm2 with PVR of 3—4 [6]. For GaAs based optoelectronic device integration, it is important to achieve good performance for RTDs grown directly on GaAs sub=
=
=
0022-0248/91/$03.50 © 1991
—
=
strates, since thick intervening strain relieving layers of other materials such as InAs can cornpromise the desired objective as InAs is optically absorbing in the long wavelength regime of interest. The key to improving the PVR of RTDs while maintaining high J~,is to use barrier and well materials with high conduction band (CB) discontinuity (~E~). In the GaAs/Al~Ga1_~Assystem while the r—r~E~ can reach 1.0 eV for x 1.0, it is the r—x 1L~E~ o f onlyeffectiveness 0.194 eV which operative thus reducing the of theisbarrier in cutting down non-resonant valley current. Use of the lower bandgap In~Ga 1_~Asfor the well region offers increasingly larger I’—Xmismatched ~ with increasing x. However, being lattice to GaAs and AlAs, it poses two severe difficulties for high quality growth. First, for x 0.5, the critical thickness for misfit (3.5%) induced defect formation, if a 2D layer-by-layer mode of growth could be maintained, is <10 monolayers for the individual layer itself. For x 0.30 there is in general a tendency for the growth mode to change over to the 3D island formation after a few monolayers of growth depending upon the growth conditions employed. Second, the large differences in
Elsevier Science Publishers BY. (North-Holland)
=
—
R.M. Kapre et al.
/ Highly strainedpseudomorphic In~,Ga, -
the surface mobilities of In, Ga, and Al as well as the congruent temperatures of InAs, GaAs and AlAs, impose severe limitations on optimizing growth conditions for simultaneous high quality growth of AlAs barriers and In~Ga1 ~As well layers. It has been demonstrated that growth of In~Ga1_~Ason pre-patterned GaAs mesas results in reduction of misfit dislocation density [7]. Consequently this technique was explored in this work on the For growth strained _~As/AIAs RTDs. RTDsof with high In In~Ga1 ( 25%), the strain appeared to be large enough for the onset of 3D island growth after the deposition of 10 monolayers (ML) under most growth conditions employed earlier [5,9]. Substrate patterning on 20 urn x 20 ~tm scale did not improve the situation. Consequently, to maintain layer-by-layer growth, new growth conditions providing better control of growth kinetics were explored and employed in the present work. In addition, we have examined the use of (InAs)M/(GaAs)N short period superlattice (SPSL) as the strained layers. This was motivated by previous work on such strain layer modulated structures [8] where good quality interfaces were realized, —
2. Resonant tunneling structures The resonant tunneling structures discussed here are of the triple-well double-barrier type consisting of undoped top and bottom In~Ga1_~As spacers, undoped AlAs barriers, and undoped In~Ga1_~Aswell. The presence of an undoped spacer well was shown in our previous work [9] to
LAs/AlAs based RTDs grown on GaAs
1111
be effective in reducing non-resonant valley current as compared to the usual undoped GaAs spacers. The undoped tunneling structures were grown on top of 0.7 ~m n~-GaAsepitaxical layers on n~-GaAs(100)substrates and capped with 0.6 ~.tm ntGaAs. RTDs with In0 1Ga09As well and spacer layers have resulted in room temperature J~, 11 kA/cm~and PVR 3.2 [9],while those with 1n025Ga075As well and spacer gaveimprovement J~, 10—20 2 and PVR 4.5 [5]. Further kA/cm x> 0.25. By optimizing MBE growth requires kinetics based upon reflection high energy electron diffraction (RHEED) pattern and intensity, we have successfully grown a series of samples with In content in the well regions from 30% to 33%. RTD device structures are summarized in table 1. RTDs No. 19 and No. 21 have a doping density ND 1 x 1018 cm while the others have ND 2 X 1018 cm3 in the contact layers. RTDs No. 19 and No. 29 have conventional homogeneous random alloy InGaAs layers while the remaining contain SPMQW layers. Growth conditions used for these RTDs are summarized in table 2. A substrate temperature increase/decrease was typically accomplished in 60/120 s by interrupting growth and rapidly adjusting the power input to the substrate heater. Rapid changes in As 4 pressure were made possible by using two As4 cells. Monolayer delivery times, TML, were determined by RHEED intensity oscillations on InAs and GaAs substrates immediately prior to the growth of the actual structures. During the RTD growth, the RHEED pattern was used to infer growth front morphology with streaky/spotty pattern indicating 2D/3D growth. The As4 pres—
=
=
=
~,
=
Table 1 Structural parameters of the RTDs reported; RTDs No. 19 and No. 29 have conventional homogeneous random alloy In~Ga1_~As layers while the others have (InAs)M/(GaAs)N short period multiple quantum well layers; the thicknesses are specified in units of monolayers (1 ML = 2.83 A) RTD No.
Spacer material
Well material
d~ (ML)
b (ML)
w (ML)
19 29 21 23 28 31
In0 3Ga0 7As In0 3Ga0 7As (InAs)1/(GaAs)2 (InAs)1/(GaAs)4 (InAs)1/(GaAs)4 (InAs)1/(GaAs)4
In0 3Ga07As In0 3Ga07As (InAs)1/(GaAs)2 (InAs)1/(GaAs)2 (InAs)1/(GaAs)2 (InAs)1/(GaAs)2
20 15 15 15 15 15
9 8 10 8 6 4
23 18 24 18 18 18
1112
/ Highly strainedpseudomorphic In~Ga,
R.M. Kapre el a!.
—
LAs/AlAs based RTDs grown on GaAs
Table 2 Growth conditions used for the RTDs reported; the substrate temperature read by a calibrated pyrometer, As
4 pressure read by a
beam flux monitor, and the monolayer delivery time,
TML,
are tabulated
RTD No.
Spacer and well layer
7.ub
~As
TML
(°C) ~ub
(10—6 Ton) ~As
(s) TMI.
Bather layer (°C)
(10—6 Ton)
(s)
19 29 21 23 28 31
555 425 475 475 425 425
6.5 2.3 2.8 4.8 1.6 1.6
2.8 2.7 4 4 4 4
555 525 600 600 525 525
6.5 3.7 7.0 4.8 3.7 3.7
3.5 4.2 3.5 2 4.2 4.2
sures for samples No. 19 No. 21, and No. 23 were chosen to maintain sufficiently As-stabilized growth conditions, since barely As-stabilized conditions were earlier found to be detrimental to RTD performance [9]. The As 4 pressures for sampies No. 28, No. 29, and No. 31 were chosen such that the As incorporation rate was about 1.5 times that of the group III elements. For this, the As incorporation rate was determined by performing As controlled RHEED oscillation experiments [10]. The SPMQW essentially equivalent to 20% and 33% alloys were grown by deposition of 1 ML of InAs followed by 4 ML or 2 ML of GaAs with a 5 to 10 s growth interruption before deposition of the next period. In order to study the effect of growth on finite substrates, a portion of the GaAs(100) substrates for RTD’s No. 19, No. 21, and No. 23 was patterned into 20 ~smx 20 urn wide and 1 urn deep mesas. The grown structures were processed into 12 urn X 12 ~im mesas with AuGe/Ni top and In back alloyed ohmic contacts. ,
3. Res~tsand discussion Fig. la shows the 77 K I—V characteristics from the patterned region of RTD No. 19 containing In 0 3Ga07As alloy in the spacer and well layers. A weak negative differential resistance (NDR) effect is seen for both polarities. The + polarity indicates top of the mesa biased positive with respect to the bottom. The non-patterned region of this sample did not show a NDR. Figs. lb and
ic show the cross-sectional TEM micrographs from the patterned and non-patterned regions. Both regions show that a 3D island mode of growth had set in and a high density of disloca9 cm2, as measured by TEM, lions of 5 X i0 —
E •~.
b C Fig. 1. (a) 77 K typical 1—V characteristics of 12 i&m ~c12 ~im size RTDs made from the patterned region of RTD No. 19 containing conventional In0 3Ga As alloys. The symbol + re. . fers to top of mesa biased positive with respect to the bottom. Cross-sectional TEM micrographs of RTD No. 19 from (b) patterned region, (c) non-patterned region.
R.M. Kapre et al.
—
/
Highly strainedpseudomorphic Jn~Ga
1— ,~As/AL4sbased RTDs grown on GaAs
O.2VIdIv
a
+
1113
b
Fig 2. (a) 300 K typical I—V characteristics of 12 ~mX12 ~&msize RTDs made from the non-patterned region of RTD No. 21 containing (lnAs)1 /(GaAs)2 SPMQW. The symbol + refers to top of mesa biased positive with respect to the bottom. (b) A cross-sectional TEM micrograph of RTD No. 21 from the non-patterned region.
I>_ 0 V
-
O.2Vk~ a.
+
—
O~2V1dIv b
+
-
C. Figs 3. 300 K typical I—V characteristics of 12 pm X 12 pm size RTDs made from the non-patterned region of (a) RTD No. 23 with wells grown at 475°C,(b) RTD No. 28 with wells grown at 425°C.Both the RTDs contain (lnAs)1/(GaAs)2 SPMQW spacer and (InAs)1/(GaAs)2 SPMQW well regions. (c) A cross-sectional TEM micrograph of RTD No. 28 (non-patterned).
1114
R.M. Kapre et aL
/ Highly strainedpseudomorphic Inja,
have propagated through the cap layer. Due to the 3D mode of growth, the advantages that accrue from patterning in the layer-by-layer growth regme of strained In~Ga 1_~Asare lostpatterning [111. Consequently, only a marginal effect of on resonant tunneling is observed. The I—V characteristics at 300 K from the non-patterned region of RTD No. 21 (<33%) In SPMQW) are shown in fig. 2a. Note (table 2) that the growth temperature has been lowered to 475 from 555°C for RTD No. 19. Here the NDR effect is stonger than for RTD No. 19, though much inferior to state-of-theart RTDs. Results from the patterned region were similar to that from the non-patterned. The improvernent over RTD No. 19 correlates well with the superior morphology observed in TEM (fig. 2b). A 3D mode of growth had set in and a dislocation density of 1 x io~ cm2 measured by TEM is observed. A large asymmetry is seen in the I—V characteristics with the improved NDR measured for tunneling from the substrate into the cap layer. This is indicative of the expected roughening of the interface with thicker growth of strained layers. To relieve the influence of increasing strain, but without compromising the relevant z~ E~ between the well and barrier layers, we examined structures with lower In composition in the spacer layer only. Fig. 3a shows the I—V characteristics from the non-patterned region of RTD No. 23 (<20%) In SPMQW spacers, <33%) —
In SPMQW layer), resultsAfrom patterned regionwell being againthe similar. muchthehigher current density at resonance is observed, consistent with the higher contact layer doping of 2 X 1018 cm3 used in this sample. However, the NDR effect is small, indicative of poor interfacial quality as is the case for RTDs No. 19 and No. 21. RHEED studies of <33~)In SPMQW indicated that a 2D layer-by-layer growth mode could be maintained to a much higher thickness at a growth temperature of 425°C as compared to 475°C. Hence RTD No. 28 was grown at 425°C with the As 4 beam equivalent pressure (BEP) reduced to 1.6 x l0 6 Torr. The AlAs barriers were grown at a temperature of 525°C to avoid deterioration of the strained films and also to reduce the growth interruption time required for temperature changes. The I—V characteristics from RTD No.
—
~As/AlAs
based RTDs grown on GaAs
28 at 300 K are shown in fig. 3b. The figure shows a of 125 (104) kA/cm2 with a PVR of 4.7 (4.7) for positive (negative) bias. At 77 K, a J~,of 128 2 with PVR of 7.6 (6.7) is seen. To (111) kA/cm our knowledge, this is the first time a high room temperature PVR has been obtained sirnultaneously with 100 kA/cm2 in RTDs grown directly on GaAs substrate without any intermediate, thick strain relieving layers. The high degree of symmetry in the I—V characteristics is indicative of good interfacial quality. The high structural quality of the RTD is also corroborated by the cross-sectional TEM micrograph shown in fig. 3c. A search over a wide area did not reveal any misfit dislocations. The excellent results obtamed for RTD No. 28 compared to RTD No. 23, which is a structure identical to it (except for a small difference in barrier thickness), indicate that the growth conditions play a major role in 3D island formation. The RHEED pattern remained streaky all the way through the growth of RTD No. 28 as opposed to RTDs No. 19, No. 21, and No. 23 where it turned spotty during the growth. RTD No. 31 was grown under conditions identical to RTD No. 28 and resulted in a 77 K J~,of 319 (250) kA/crn2 with PVR of 2.3 (2.12) for positive (negative) bias. Due to high power dissipation in RTD No. 31, when tested at room temperature, the devices were destroyed. RTD No. 29 with conventional alloy In 203Ga07As with PVRlayers of 4.7showed (5.2) fora J~, o f 32 (32) kA/cm positive (negative) bias at room temperature. The growth conditions were identical to RTD No. 28.
Acknowledgements This work was supported by the Air Force Office of Scientific Research, the Joint Services Electronics Program at the University of Southern California, and the Office of Naval Research.
References [1] T.P.E. Broekart W. Lee and C.G. Fonstad, Appl. Phys. utters 53 (1988) 1545. [2] T.P.E. Broekart and C.G. Fonstad, in: IEEE Intern. Electron Devices Meeting Tech. Digest, 1989, p. 559.
R.M. Kapre et al.
/ Highly strainedpseudomorphic In~,Ga1 -
[3] P. Cheng and J. Harris, Appl. Phys. Letters 56 (1990) 1676. [4] S.K. Diamond, E. Ozbay, M.J.W. Rodwell, D.M. Bloom, Y.C. Pao, E. Wolak and J. Harris, IEEE Electron Device Letters EDL-10 (1989) 104. [5] R. Kapre, A. Madhukar and S. Guha, IEEE Electron Device Letters EDL-11 (1990) 270. [6] J.R. Soderstrom, T.C. McGill, E.R. Brown, C.D. Parker and U. Mahoney, presented at Electronic Materials Coaf., Santa Barbara, CA, June 1990. [7] S. Guha, A. Madhukar, K. Kaviani and R. Kapre, J. Vacuum Sci. Technol. B8 (1990) 149.
LAs/AlAs based RTDs grown on GaAs
1115
[8] M.Y. Yen, A. Madhukar, B.F. Lewis, R. Fernandez, L. Eng and FJ. Grunthaner, Surface Sci. 174 (1986) 606. [9] R. Kapre, A. Madhukar, K. Kaviani, S. Guha and K.C. Rajkumar, Appi. Phys. Letters 56 (1990) 922. [10] B.F. Lewis, R. Fernandez, A. Madhukar and F.J. Grunthaner, J. Vacuum Sci. Technol. B4 (1986) 560. [11] S. Guha, A. Madhukar, Li Chen, K.C. Rajkumar and R. Kapre, in: Proc. Society of Photo-Optical Instrumentation Engineers, Bellinghani, WA, Vol. 125, in press.
Journal of Crystal Growth 111 (1991) 1110—1115 North-Holland
Highly strained pseudomorphic In~Ga1_~As/AIAs based resonant tunneling diodes grown on patterned and non-patterned GaAs( 100) substrates R.M. Kapre, A. Madhukar and S. Guha Photonic Materials and Devices Laboratory, University of Southern California; Los Angeles, California 90089-0241, USA
Resonant tumieling diodes (RTDs) with strained well and spacer regions made of In~Ga1_~Asalloys or (InAs)M/(GaAs)N short period multiple quantum well (SPMQW) have been fabricated on GaAs(100) substrates. With increasing x, the presence of strain results in generation of misfit dislocations and/or change to three-dimensional island mode of growth, both detrimental to the performance of the RTDs. Consequently, a systematic study of the electrical and structural properties of RTDs with 0.10 x 0.33 was carried out with emphasis on control of the growth kinetics. This has led to RTDs with (InAs)1/(GaAs)2 room 2. This is the firstwells timehaving that devices grown on GaAs any thick strain relieving intermediate layers shown simultaneously high PVRs and J~,. temperature peaksubstrate to valleywithout ratios (PVR) of 4.7 with peak current densities (Jr)have of 125 kA/cm
1. Introduction Resonant tunneling diodes (RTDs) have been fabricated in various material systems including GaAs/A1GaAs, In 0 53Ga047As/ALks, and InAs/ A1Sb grown on GaAs, InP, and InAs substrates, respectively. Considering the usual figures of merit of peak current density (Jr) and peak-to-valley ratio (PVR), strained RTDs with In0 53Ga047As/ InAs wells and AlAs barriers grown on InP substrate have shown the best results: a room 2 [1]temperand a ature PVR of 30 withJ~, 400—500 6 kA/cmkA/cm2 [2]. PVR of 3—4 with Lattice matched GaAs/ A1GaAs RTDs on =
=
GaAs(100) substrates have shown room tempera2 [3] ture PVR PVRsofof 4—5with with J~,100—200 10—20 kA/cm2 kA/cm [4]. and 2—2.5 Strained InGaAs/AIAs RTDs on GaAs(100) have shown comparable behaviour with PVR of 4.7 and 10—12 kA/cm2 [5].InAs/A1Sb devices based on thick n~-InAs epilayers on top of GaAs (i.e. very low strain in the RTD) have yielded room temperature J~, 200—400 kA/cm2 with PVR of 3—4 [6]. For GaAs based optoelectronic device integration, it is important to achieve good performance for RTDs grown directly on GaAs sub=
=
=
0022-0248/91/$03.50 © 1991
—
=
strates, since thick intervening strain relieving layers of other materials such as InAs can cornpromise the desired objective as InAs is optically absorbing in the long wavelength regime of interest. The key to improving the PVR of RTDs while maintaining high J~,is to use barrier and well materials with high conduction band (CB) discontinuity (~E~). In the GaAs/Al~Ga1_~Assystem while the r—r~E~ can reach 1.0 eV for x 1.0, it is the r—x 1L~E~ o f onlyeffectiveness 0.194 eV which operative thus reducing the of theisbarrier in cutting down non-resonant valley current. Use of the lower bandgap In~Ga 1_~Asfor the well region offers increasingly larger I’—Xmismatched ~ with increasing x. However, being lattice to GaAs and AlAs, it poses two severe difficulties for high quality growth. First, for x 0.5, the critical thickness for misfit (3.5%) induced defect formation, if a 2D layer-by-layer mode of growth could be maintained, is <10 monolayers for the individual layer itself. For x 0.30 there is in general a tendency for the growth mode to change over to the 3D island formation after a few monolayers of growth depending upon the growth conditions employed. Second, the large differences in
Elsevier Science Publishers BY. (North-Holland)
=
—
R.M. Kapre et al.
/ Highly strainedpseudomorphic In~,Ga, -
the surface mobilities of In, Ga, and Al as well as the congruent temperatures of InAs, GaAs and AlAs, impose severe limitations on optimizing growth conditions for simultaneous high quality growth of AlAs barriers and In~Ga1 ~As well layers. It has been demonstrated that growth of In~Ga1_~Ason pre-patterned GaAs mesas results in reduction of misfit dislocation density [7]. Consequently this technique was explored in this work on the For growth strained _~As/AIAs RTDs. RTDsof with high In In~Ga1 ( 25%), the strain appeared to be large enough for the onset of 3D island growth after the deposition of 10 monolayers (ML) under most growth conditions employed earlier [5,9]. Substrate patterning on 20 urn x 20 ~tm scale did not improve the situation. Consequently, to maintain layer-by-layer growth, new growth conditions providing better control of growth kinetics were explored and employed in the present work. In addition, we have examined the use of (InAs)M/(GaAs)N short period superlattice (SPSL) as the strained layers. This was motivated by previous work on such strain layer modulated structures [8] where good quality interfaces were realized, —
2. Resonant tunneling structures The resonant tunneling structures discussed here are of the triple-well double-barrier type consisting of undoped top and bottom In~Ga1_~As spacers, undoped AlAs barriers, and undoped In~Ga1_~Aswell. The presence of an undoped spacer well was shown in our previous work [9] to
LAs/AlAs based RTDs grown on GaAs
1111
be effective in reducing non-resonant valley current as compared to the usual undoped GaAs spacers. The undoped tunneling structures were grown on top of 0.7 ~m n~-GaAsepitaxical layers on n~-GaAs(100)substrates and capped with 0.6 ~.tm ntGaAs. RTDs with In0 1Ga09As well and spacer layers have resulted in room temperature J~, 11 kA/cm~and PVR 3.2 [9],while those with 1n025Ga075As well and spacer gaveimprovement J~, 10—20 2 and PVR 4.5 [5]. Further kA/cm x> 0.25. By optimizing MBE growth requires kinetics based upon reflection high energy electron diffraction (RHEED) pattern and intensity, we have successfully grown a series of samples with In content in the well regions from 30% to 33%. RTD device structures are summarized in table 1. RTDs No. 19 and No. 21 have a doping density ND 1 x 1018 cm while the others have ND 2 X 1018 cm3 in the contact layers. RTDs No. 19 and No. 29 have conventional homogeneous random alloy InGaAs layers while the remaining contain SPMQW layers. Growth conditions used for these RTDs are summarized in table 2. A substrate temperature increase/decrease was typically accomplished in 60/120 s by interrupting growth and rapidly adjusting the power input to the substrate heater. Rapid changes in As 4 pressure were made possible by using two As4 cells. Monolayer delivery times, TML, were determined by RHEED intensity oscillations on InAs and GaAs substrates immediately prior to the growth of the actual structures. During the RTD growth, the RHEED pattern was used to infer growth front morphology with streaky/spotty pattern indicating 2D/3D growth. The As4 pres—
=
=
=
~,
=
Table 1 Structural parameters of the RTDs reported; RTDs No. 19 and No. 29 have conventional homogeneous random alloy In~Ga1_~As layers while the others have (InAs)M/(GaAs)N short period multiple quantum well layers; the thicknesses are specified in units of monolayers (1 ML = 2.83 A) RTD No.
Spacer material
Well material
d~ (ML)
b (ML)
w (ML)
19 29 21 23 28 31
In0 3Ga0 7As In0 3Ga0 7As (InAs)1/(GaAs)2 (InAs)1/(GaAs)4 (InAs)1/(GaAs)4 (InAs)1/(GaAs)4
In0 3Ga07As In0 3Ga07As (InAs)1/(GaAs)2 (InAs)1/(GaAs)2 (InAs)1/(GaAs)2 (InAs)1/(GaAs)2
20 15 15 15 15 15
9 8 10 8 6 4
23 18 24 18 18 18
1112
/ Highly strainedpseudomorphic In~Ga,
R.M. Kapre el a!.
—
LAs/AlAs based RTDs grown on GaAs
Table 2 Growth conditions used for the RTDs reported; the substrate temperature read by a calibrated pyrometer, As
4 pressure read by a
beam flux monitor, and the monolayer delivery time,
TML,
are tabulated
RTD No.
Spacer and well layer
7.ub
~As
TML
(°C) ~ub
(10—6 Ton) ~As
(s) TMI.
Bather layer (°C)
(10—6 Ton)
(s)
19 29 21 23 28 31
555 425 475 475 425 425
6.5 2.3 2.8 4.8 1.6 1.6
2.8 2.7 4 4 4 4
555 525 600 600 525 525
6.5 3.7 7.0 4.8 3.7 3.7
3.5 4.2 3.5 2 4.2 4.2
sures for samples No. 19 No. 21, and No. 23 were chosen to maintain sufficiently As-stabilized growth conditions, since barely As-stabilized conditions were earlier found to be detrimental to RTD performance [9]. The As 4 pressures for sampies No. 28, No. 29, and No. 31 were chosen such that the As incorporation rate was about 1.5 times that of the group III elements. For this, the As incorporation rate was determined by performing As controlled RHEED oscillation experiments [10]. The SPMQW essentially equivalent to 20% and 33% alloys were grown by deposition of 1 ML of InAs followed by 4 ML or 2 ML of GaAs with a 5 to 10 s growth interruption before deposition of the next period. In order to study the effect of growth on finite substrates, a portion of the GaAs(100) substrates for RTD’s No. 19, No. 21, and No. 23 was patterned into 20 ~smx 20 urn wide and 1 urn deep mesas. The grown structures were processed into 12 urn X 12 ~im mesas with AuGe/Ni top and In back alloyed ohmic contacts. ,
3. Res~tsand discussion Fig. la shows the 77 K I—V characteristics from the patterned region of RTD No. 19 containing In 0 3Ga07As alloy in the spacer and well layers. A weak negative differential resistance (NDR) effect is seen for both polarities. The + polarity indicates top of the mesa biased positive with respect to the bottom. The non-patterned region of this sample did not show a NDR. Figs. lb and
ic show the cross-sectional TEM micrographs from the patterned and non-patterned regions. Both regions show that a 3D island mode of growth had set in and a high density of disloca9 cm2, as measured by TEM, lions of 5 X i0 —
E •~.
b C Fig. 1. (a) 77 K typical 1—V characteristics of 12 i&m ~c12 ~im size RTDs made from the patterned region of RTD No. 19 containing conventional In0 3Ga As alloys. The symbol + re. . fers to top of mesa biased positive with respect to the bottom. Cross-sectional TEM micrographs of RTD No. 19 from (b) patterned region, (c) non-patterned region.
R.M. Kapre et al.
—
/
Highly strainedpseudomorphic Jn~Ga
1— ,~As/AL4sbased RTDs grown on GaAs
O.2VIdIv
a
+
1113
b
Fig 2. (a) 300 K typical I—V characteristics of 12 ~mX12 ~&msize RTDs made from the non-patterned region of RTD No. 21 containing (lnAs)1 /(GaAs)2 SPMQW. The symbol + refers to top of mesa biased positive with respect to the bottom. (b) A cross-sectional TEM micrograph of RTD No. 21 from the non-patterned region.
I>_ 0 V
-
O.2Vk~ a.
+
—
O~2V1dIv b
+
-
C. Figs 3. 300 K typical I—V characteristics of 12 pm X 12 pm size RTDs made from the non-patterned region of (a) RTD No. 23 with wells grown at 475°C,(b) RTD No. 28 with wells grown at 425°C.Both the RTDs contain (lnAs)1/(GaAs)2 SPMQW spacer and (InAs)1/(GaAs)2 SPMQW well regions. (c) A cross-sectional TEM micrograph of RTD No. 28 (non-patterned).
1114
R.M. Kapre et aL
/ Highly strainedpseudomorphic Inja,
have propagated through the cap layer. Due to the 3D mode of growth, the advantages that accrue from patterning in the layer-by-layer growth regme of strained In~Ga 1_~Asare lostpatterning [111. Consequently, only a marginal effect of on resonant tunneling is observed. The I—V characteristics at 300 K from the non-patterned region of RTD No. 21 (<33%) In SPMQW) are shown in fig. 2a. Note (table 2) that the growth temperature has been lowered to 475 from 555°C for RTD No. 19. Here the NDR effect is stonger than for RTD No. 19, though much inferior to state-of-theart RTDs. Results from the patterned region were similar to that from the non-patterned. The improvernent over RTD No. 19 correlates well with the superior morphology observed in TEM (fig. 2b). A 3D mode of growth had set in and a dislocation density of 1 x io~ cm2 measured by TEM is observed. A large asymmetry is seen in the I—V characteristics with the improved NDR measured for tunneling from the substrate into the cap layer. This is indicative of the expected roughening of the interface with thicker growth of strained layers. To relieve the influence of increasing strain, but without compromising the relevant z~ E~ between the well and barrier layers, we examined structures with lower In composition in the spacer layer only. Fig. 3a shows the I—V characteristics from the non-patterned region of RTD No. 23 (<20%) In SPMQW spacers, <33%) —
In SPMQW layer), resultsAfrom patterned regionwell being againthe similar. muchthehigher current density at resonance is observed, consistent with the higher contact layer doping of 2 X 1018 cm3 used in this sample. However, the NDR effect is small, indicative of poor interfacial quality as is the case for RTDs No. 19 and No. 21. RHEED studies of <33~)In SPMQW indicated that a 2D layer-by-layer growth mode could be maintained to a much higher thickness at a growth temperature of 425°C as compared to 475°C. Hence RTD No. 28 was grown at 425°C with the As 4 beam equivalent pressure (BEP) reduced to 1.6 x l0 6 Torr. The AlAs barriers were grown at a temperature of 525°C to avoid deterioration of the strained films and also to reduce the growth interruption time required for temperature changes. The I—V characteristics from RTD No.
—
~As/AlAs
based RTDs grown on GaAs
28 at 300 K are shown in fig. 3b. The figure shows a of 125 (104) kA/cm2 with a PVR of 4.7 (4.7) for positive (negative) bias. At 77 K, a J~,of 128 2 with PVR of 7.6 (6.7) is seen. To (111) kA/cm our knowledge, this is the first time a high room temperature PVR has been obtained sirnultaneously with 100 kA/cm2 in RTDs grown directly on GaAs substrate without any intermediate, thick strain relieving layers. The high degree of symmetry in the I—V characteristics is indicative of good interfacial quality. The high structural quality of the RTD is also corroborated by the cross-sectional TEM micrograph shown in fig. 3c. A search over a wide area did not reveal any misfit dislocations. The excellent results obtamed for RTD No. 28 compared to RTD No. 23, which is a structure identical to it (except for a small difference in barrier thickness), indicate that the growth conditions play a major role in 3D island formation. The RHEED pattern remained streaky all the way through the growth of RTD No. 28 as opposed to RTDs No. 19, No. 21, and No. 23 where it turned spotty during the growth. RTD No. 31 was grown under conditions identical to RTD No. 28 and resulted in a 77 K J~,of 319 (250) kA/crn2 with PVR of 2.3 (2.12) for positive (negative) bias. Due to high power dissipation in RTD No. 31, when tested at room temperature, the devices were destroyed. RTD No. 29 with conventional alloy In 203Ga07As with PVRlayers of 4.7showed (5.2) fora J~, o f 32 (32) kA/cm positive (negative) bias at room temperature. The growth conditions were identical to RTD No. 28.
Acknowledgements This work was supported by the Air Force Office of Scientific Research, the Joint Services Electronics Program at the University of Southern California, and the Office of Naval Research.
References [1] T.P.E. Broekart W. Lee and C.G. Fonstad, Appl. Phys. utters 53 (1988) 1545. [2] T.P.E. Broekart and C.G. Fonstad, in: IEEE Intern. Electron Devices Meeting Tech. Digest, 1989, p. 559.
R.M. Kapre et al.
/ Highly strainedpseudomorphic In~,Ga1 -
[3] P. Cheng and J. Harris, Appl. Phys. Letters 56 (1990) 1676. [4] S.K. Diamond, E. Ozbay, M.J.W. Rodwell, D.M. Bloom, Y.C. Pao, E. Wolak and J. Harris, IEEE Electron Device Letters EDL-10 (1989) 104. [5] R. Kapre, A. Madhukar and S. Guha, IEEE Electron Device Letters EDL-11 (1990) 270. [6] J.R. Soderstrom, T.C. McGill, E.R. Brown, C.D. Parker and U. Mahoney, presented at Electronic Materials Coaf., Santa Barbara, CA, June 1990. [7] S. Guha, A. Madhukar, K. Kaviani and R. Kapre, J. Vacuum Sci. Technol. B8 (1990) 149.
LAs/AlAs based RTDs grown on GaAs
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[8] M.Y. Yen, A. Madhukar, B.F. Lewis, R. Fernandez, L. Eng and FJ. Grunthaner, Surface Sci. 174 (1986) 606. [9] R. Kapre, A. Madhukar, K. Kaviani, S. Guha and K.C. Rajkumar, Appi. Phys. Letters 56 (1990) 922. [10] B.F. Lewis, R. Fernandez, A. Madhukar and F.J. Grunthaner, J. Vacuum Sci. Technol. B4 (1986) 560. [11] S. Guha, A. Madhukar, Li Chen, K.C. Rajkumar and R. Kapre, in: Proc. Society of Photo-Optical Instrumentation Engineers, Bellinghani, WA, Vol. 125, in press.