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OMCVD growth of InP, InGaAs, and InGaAsP on (110) InP substrates
Journal of Crystal Growth 124 (1992) 311—317 North-Holland
o~o,
CRYSTAL OROWT H
OMCVD growth of InP, InGaAs, and InGaAsP on (110) InP substrates R. Bhat, M.A. Koza, D.M. Hwang, M.J.S.P. Brasil, R.E. Nahory Bellcore, 331 Newman Springs Road, Red Bank, New Jersey 07701-7040, USA
and K.Oe NTT Optoelectronics Laboratory, Atsugi 243-01, Japan
Device quality InP and InGaAsP (Ag = 1.0—1.55 ~sm) have been grown on 3~off (110) towards (111)B InP substrates. Shallow ledges running along the [1101direction form when the layer thickness is > 1 ~sm, indicating that a higher degree of misorientation is desirable. InGaAs showed severe facetting, even for thin layers, when grown on (110) substrates misoriented towards (111)B and a granular feature when grown on those misoriented towards (111)A. The InGaAsP layers have room temperature photoluminescence characteristics which are comparable to or better than those grown on (100) substrates. In addition, InGaAsP quantum wells, with compressive and tensile strain, have room temperature photoluminescence intensities which are 2—5 x higher than those 2 was obtained for a 3 quantum obtained (100) substrates. well laseron with a cavity lengthInofpreliminary 1.2 mm. experiments, a threshold current density of 1.5 kA/cm
1. Introduction
on (100) oriented substrates. Recently, there has
Epitaxial crystal growth studies have concentrated on the growth on (100) oriented substrates due to the fact that the natural cleavage planes are normal to the (100) crystal face and to each other, the ease with which epitaxial layers with excellent surface morphology can be grown, and the high quality of the devices fabricated with these layers. Epitaxial layers on substrates of other orientations, such as (110), can be advantageous for making use of the anisotropic properties of Ill—V compound crystals. The benefits of fabricating avalanche diodes [1] and optical modulators [2,3] in (110) GaAs or InP have been demonstrated. Quantum wells and superlattices have been intensively investigated in the past decade, due to their many unique physical properties, and potential application to novel devices. Again, most of these investigations have concentrated on growth
been increasing interest in the properties of quantum wells grown on substrates of unconventional orientations such as (110) [41and (111) [5,61,because both the energies and the oscillator strengths of the interband optical transitions are sensitive to the crystallographic direction of the epitaxial growth. In particular, studies on (110) oriented quantum wells are interesting, since the lower symmetry of the quantum confinement resuits in in-plane polarization anisotropy of the optical transition [4]. Liquid phase epitaxy (LPE) [2],molecular beam epitaxy (MBE) [3—7],and gas-source molecular beam epitaxy (GSMBE) [81 have been used for the successful growth of specular, device quality layers on (110) substrates. There are a few reports of GaAs growth by organometallic chemical vapor deposition (OMCVD) on (110); however, good morphology was only reported by Okamoto et al. [12],although some facetting is seen in their
0022-0248/92/$05.00 © 1992
—
Elsevier Science Publishers B.V. All rights reserved
312
R. Bhat et al.
/
OMCVD growth of InP, InGaAs, and !nGaAsP on (110) lnP substrates
2. Experimental procedure
Fig. 1. Typical morphology of an InP epitaxial layer on an exact (110) InP substrate. Marker represents 50 ~sm.
The growths were carried out in a horizontal, low pressure (76 Torr) OMCVD reactor using trimethyl alkyis of gallium (TMG) and indium (TM!), arsine, phosphine, and hydrogen as a carrier gas. n-Type and p-type doping was accomplished using hydrogen sulfide and diethyizinc, respectively. The substrates were tin (n 1 x 1018 cm3) or sulfur (n 5 x 1018 cm3) doped InP having a orientation of either (110) or 3°off (110) towards (11 1)A/B (hitherto designated as (1 10)A and (110)B, respectively). The misorientation direction was chosen based on the work of Allen et al. [7] for MBE of GaAs and that of Zamkotsian et al. [8] for GSMBE of InP based materials. 3. Results
layers. Furthermore, we are not aware of any
report of (110) quantum wells grown by OMCVD. In addition, there appears to be no report of any growth on (110) InP by OMCVD, except for that of GaAs 05Sb05 [13],where poor morphology was obtained. In this work, we report on the growth of InP, InGaAs, and InGaAsP on (110) and 3°off (110) towards (111)A/B InP substrates by OMCVD.
3.1. InP growth
InP growth was studied by varying the temperature in the range 550—675°C, the phosphine mole fraction from 4.2 x iO~to 3.3 x 10-2 and the TM! mole fraction from 1.7 X iO~to 8.75 X iO~.The layers on exact (110) substrates were highly facetted. A typical morphology is shown in
Fig. 2. Typical morphology of a 5 ~smthick InP layer on (a) a (110)B InP substrate showing the shallow ledges running along [110] and (b) a (110)A InP substrate showing the granular feature. Markers represent 10 ~sm.
R. Bhat eta!.
/ OMCVD growth of InP,
InGaA.~,and InGa.4sP on (110) InP substrates
fig. 1. At the rounded edges of the (110) substrates corresponding to tilting of the surface towards either the (111)A or B directions the facetting disappeared for some growth conditions. Consistent with this observation, highly specular, defect free surfaces were obtained on (110)A/B in the temperature range 550—575°C and TM! and phosphine mole fractions of 8.75 x iO~and ~ 1.6 x 10—2, respectively, resulting in a growth rate of 1.6 nm/s. A very fine granular feature was barely observable at high magnification (1000 x) under a Nomarski interference contrast microscope for (110)A but not (110)B substrates. For thick (— 5 ~m) layers, very shallow ledges running along the [110] direction appear on (110)B substrates (fig. 2a). This suggests that a misorientation an ~legreater than 3°would lead to a better morphology. For layers on (110)A substrates, the granular feature becomes better defined (fig. 2b). The background net donor concentration in these films was 3.5 x 1014 and 1 X 1015 cm3 on (110)A and (110)B substrates, re‘~
-
_______________________________
lOIS
1018
•.(100) x.(l1O)B (110) A
-
575°C TMI.8.75x105 PH 3. ~
1017
.
10~ x /
i0~~
z
10 0
/
.
____________________________ 2 4 6 8 10 12
spectively, as obtained by C—V measurements. Layers grown under the same conditions on (100)InP had a background net donor concentration of 3 x 1014 cm3. Hall measurements were not done since semi-insulating substrates were not available. A comparison of n-type doping, using hydrogen sulfide, was made by simultaneous growth on (100) and (1 10)A/B substrates. It can be seen in fig. 3 that n-doping is as much as 14 x more efficient on (110)B and 80 x less efficient on (110)A as compared to that on (100). The significantly higher incorporation of the n-type dopant on (110)B as compared to that on (110)A is consistent with the earlier work of Bhat et a. [14]. A similar comparison was also made of p-type doping using diethylzinc. The results are shown in fig. 4. It can be seen that Zn doping is as much as 4.5 x and 6 x times more efficient on (110)B and (110)A, respectively, as compared to that on (100). The higher acceptor incorporation efficiency on (110)A as compared to that on (110)B is again consistent with the earlier work of Bhat et al. [14]. 32 I GaAs
—
1O’~
40
H2S MOLE FRACTION — Fig. 3. Dependence of net donor concentration on H2S mole fraction for InP epitaxial InPlayers substrates. on (100), (110)A, and (110)B
313
n
dl Ga.AP an
n
th
s grow
As in the case of InP, the optimum growth temperature for InGaAs was found to be in the range 500—575°C. High mole fractions of TMI, TMG, and arsine were again used to obtain the best morphology and were 8.75 x iO~ 9.5 x iO~,and 8.3 x iO~,respectively, resulting in a growth rate of 3.0 nm/s. Layers grown on exact (110) substrates were highly facetted. However, unlike in the case of !nP, layers (<1 p~m thick) grown on (1 10)B showed facetting with ledges running along [110].The layers on (110)A were smoother with a slight graininess being barely visible under Nomarski interference contrast at high magnification (1000 x). These features become enhanced upon the growth of thick (5 ~m) layers as shown in figs. 5a and 5b. This again suggests that the misorientation angle is insufficient. The background net donor concentrations in these 3 films for (110)A were (2—3) and (110)B, x 10’~and respec(1 1.5) x i0’~cm tively.
314
R. Bhat eta!.
/
OMCVD growth of JnP, !nGaAs, and InGaAsP on (110) lnP substrates Table 1
1020 • .
(100)
x
(110) A
.
TMI -8.75 x iO~
-(110) B
PH
3 -1.6 x
io-~
2
8
~
InP substrates Bandgap wavelength (/Lm)
Linewidth (meV) (100) (110)
Intensity (100)
(110)
1.0 1.1
40.6 54
49.5 40.9
500 198
250 200
1.55
56.4
55.4
160
200
are nearly the same on both (110)A/B. Since the
10~~
/ / I l0’7~
Room temperature photoluminescence intensity (arbitrary units) and linewidths of InGaAsP alloys on (110)B and (100)
~
h
~
~
DEZ MOLE FRACTION —
Fig. 4. Dependence of net acceptor concentration on DEZ mole fraction for InP epitaxial layers on (100), (110)A, and (110)B InP substrates.
reactor was not optimized for uniform growth at 575°C, it is not possible to say if there is a systematic difference in the compositions obtamed on (110)A/B. However, specular morphologies, as seen using Nomarski interference contrast microscopy, were obtained only for (1 10)B, while layers on (1 10)A showed a granular surface (although it appeared specular to the naked eye). Depending on the thickness of the .
epitaxial layers, ledges running along [1101were occasionally seen, as in the case of InP. The
A range of InGaAsP compositions correspond-
layers were characterized using room tempera-
ing to bandgap wavelengths ranging from 1.0 to
ture (RT) photoluminescence (PL). In table 1 a
1.55 j~mhave been grown, lattice matched to InP, under the conditions optimized for InP and InGaAs, at 575°C.The compositions of the layers
comparison is made between the PL intensities and linewidths obtained for layers on (110)B and
(100). The layers on (100) were grown at 625°C
Fig. 5. Typical morphology of 5 ~m thick InGaAs epitaxiai layers on (a) (110)A and (b) (110)B InP substrates. Markers represent 10 ~sm.
R. Bhat eta!.
/
OMCVD growth of InP, !nGaAs, and InGaAsP on (110) InP substrates
and at approximately half the growth rate on (110), which are our optimized growth conditions for (100). It is seen that layers on (110)B are superior, with the exception of the Ag = 1.0 ~m quaternary, to those on (100). Considering the much larger data base we have on quaternaries on (100) it can be concluded that the RTPL characteristics of quaternaries on (110)B are cornparable to or better than those on (100). 3.3. InGaAs and InGaAsP quantum wells
The optimized growth conditions on (110)B substrates were used to grow single quantum wells (QWs) of InGaAs with nominal thicknesses of 2.5, 6.0, and 12.0 nm in one growth step. A thick (—~0.4 kin) reference !nGaAs layer was also grown. The PL spectrum at 5 K is shown in fig. 6. The figure shows 4 peaks corresponding to recombination in the thick InGaAs layer and in each of the 3 QW layers. It can be seen that the linewidths are comparable to those reported for (100) (17). Cross-section transmission microscopy (TEM) examination of these QWs shows (Fig. 7) that, whereas the InP layers are highly planar, the InGaAs surface breaks up into (110) treads and (111) risers, as may be expected on a (110) substrate misoriented by 3°towards the (111)B. This causes a fluctuation in the well thickness. The risers are only 2.3 nm high even for the 0.4 km thick reference layer. It should be noted here ‘—
315
that PL linewidths alone are not sufficient to judge the quality of QWs. The tendency for the InGaAs layers to facet and the planar growth front seen for !nP in fig. 7 are consistent with our observations on thick InP and InGaAs layer surface morphologies. In fig. 7 it can also be seen that the growth of InP on the facetted InGaAs rapidly planarizes the surface. Since the surface morphologies of quaternary layers (Ag = 1.0—1.55 p~m)were smooth and planar, several InGaAsP multiple QW layer structures with InGaAsP (Ag = 1.3 p~mfor compressive and 1.2 ~.amfor tensile strained wells) barrier layers were grown with a view towards 1.55 ~sm laser fabrication. The OW layers were 6 nm thick In062Ga038As086P014 (4a/a = + 0.17%), 2.4 nm thick In0 73Ga027As096P004 (6a/a = 14 nm thick In039Ga061As086P014 (ôa/a = 1.5%). Indeed, as anticipated, crosssection TEM examination of the compressive strained QWs indicated that the layers were planar. However, the tensile strained QWs were facetted as in the case of InGaAs QWs. Therefore, it appears that not only the As/P ratio but also the In/Ga ratio influences the formation of facets. The PL intensity from the compressive strained QWs was found to be 2—5 x higher than that for the best quantum wells grown by us on (100). + 0.94%), or —
‘~
4. Discussion 5K
1525.Onm
I
I
-
~E-6meV
1368~Onm 1254.0 nm ~7N.~-18 1200.00
~
i~so.onm
~E-9.3maY
1400.00 Wavelength, nm
~E-9.1maY 1600.00
Fig. 6. 5 K PL spectrum of InGaAs single quantum wells with nominal thicknesses of 2.5, 6.0, and 12 nm. The peak at 1525 nm is from the 0.4 ~im thick InGaAs reference layer.
The (110) orientation has traditionally been viewed as a difficult one for epitaxial growth because nucleation is difficult [151. Atoms on {110} surfaces form planar zigzag chains, each atom being bonded to two adjacent atoms in the chain. Each surface atom has one dangling bond. Therefore, the first atom depositing to start a new layer can make only one bond to the existing surface and three dangling bonds are created, an increase of two dangling bonds. Hence there is a potential barrier for nucleation. Once the first atom is in place however nucleation can proceed by the addition of atoms of the opposite type alternately at each end of the bonding chain without further nucleation barriers. The probabil-
316
R. Bhat eta!.
/
OMCVD growth of InP, InGaAs, and InGaAsP on (110) InP substrates
-~
Fig. 7. Cross-section TEM micrograph of InGaAs single quantum wells on a (110)B InP substrate. The dark bands are the InP barrier layers. The polycrystalline layer on the surface is an inadvertently grown GaP layer. The InGaAs reference layer shows well-developed facets.
ity of Ill—V interaction is increased when both III and V concentrations are high on the surface, being proportional to the product of the concentrations. It can also be argued that the growth temperature should be low so as to enhance the sticking coefficient of the V element. However, it cannot be so low as to significantly lower the cracking efficiency of the V hydride. These arguments are consistent with our experimental resuits wherein high III and V reagent mole fractions and low growth temperatures are found to be necessary for good epitaxy on (110). Allen et al. [7,16]found that in MBE of GaAs smooth epitaxial layers were obtained only on (110)A. They argue that misorientations towards (111)A produces stable (111)A ledges upon which As can nucleate and growth can proceed in a layer-by-layer mode. Zamkotsian et al. [8] were
able to obtain specular InP and InGaAsP layers on both (110)A and B using GSMBE, although it is not clear if one orientation is better than the other. In our work, it was found that misorientations towards (11 1)B gave superior layers of !nP and InGasAsP but InGaAs grew better on substrates misoriented towards (111)A. However, the surface morphoiogies of thick (5 ~sm) InP and InGaAs layers on (110)B suggest that a higher degree of misorientation towards (111)B may produce more planar InP and InGaAs surfaces. 5. Lasers A separate confinement heterostructure laser structure, with the active layer consisting of three 6 nm thick !n062Ga038As096P0~(Ag = 1.64 /Lm)
R. Bhat et aL
/ OMCVD growth of InP, InGaAs,
quantum wells and 20 nm thick InGaAsP (Ag = 1.3 ~sm) barriers was processed into 50 ~m wide oxide stripe broad area lasers. The threshold current density for lasers with a cavity length of 1.2 mm was 1.5 kA/cm2, which is comparable to the GSMBE results of Zamkotsian et al. [8].With further optimization of the growth and laser structures and utilization of compressive and tensue strain in the quantum wells, substantial reduction in the threshold current density is anticipated. 6. Conclusions The OMCVD growth of device quality InP based materials on (1 10)B substrates has been achieved. Further improvements in the surface morphology, particularly of !nGaAs and of thick !nP, may be expected for layers grown on substrates with larger misorientations. The doping behaviour of sulfur and zinc in (110) InP has been established. It is found that the room temperature photoluminescence of InGaAsP grown on (110)B is comparable to or better than those on (100). Similarly, quantum wells on (110) have a higher PL intensity than those on (100). Finally, preliminary results indicate that quantum well lasers on (110)B are promising.
and InGaAsP on (110) InP substrates
317
[2] J. McKenna and F.K. Reinhart, J. Appl. Phys. 47 (1976) 2069. [3] F. Zamkotsian, C. Artigue, J-L. Peyre, M. Lambert, D. Bonnevie and J. Benoit, Electron. Letters 26 (1990) 1560. [4] D. Gershoni, I. Brener, GA. Baraff, S.N.G. Chu, L.N. Pfeiffer and K. West, Phys. Rev B 44 (1991) 1930. [5] T. Hayakawa, M. Kondo, T. Suyama, K. Takahashi, S. Yamamoto and T. Hijikata, Japan. J. AppI. Phys. 26 (1987)L302. [6] L.W. Molenkamp, R. Eppenga, G.W. ‘t Hooft, P. Dawson, C.T. Foxon and KJ. Moore, Phys. Rev. B 38 (1988) 4314. [7] L.T.P. Allen, ER. Weber, J. Washburn and Y.C. Pao, Appl. Phys. Letters 51(1987) 670. [8] F. Zamkotsian, F. Poingt, L. Le Gouezigou, F. Gaborit, J.-G. Provost, C. Artigue and J. Benoit, in: Proc. Conf. on InP and Related Materials, 1991, p. 584. [9] V. Gottschalck, W.-H. Petzke and E. Butter, Kristall Tech. 9 (1974) 209. [10] D.H. Reep and S.K. Ghandhi, J. Crystal Growth 61 (1983) 449. [11] K. Kamon, M. Shimazu, K. Kimura, M. Mihara and M. Ishi, J. Crystal Growth 84 (1987) 126. [12] K. Okamoto, M. Furuta and K. Yamaguchi, Japan. J. Appl. Phys. 27 (1988) L2121. [13] H.R. Jen, M.J. Jou, Y.T. Cherng and G.B. Stringfellow, J. Crystal Growth 85 (1987) 175. [14] R. Bhat, C. Caneau, C.E. Zah, M.A. Koza, W.A. Bonner, P.M. Hwang, S.A. Schwarz, S.G. Menocal and F.G. Favire, J. Crystal Growth 107 (1991) 772. [15] R.C. Sangster, in: Compound Semiconductors, Vol. 1, Preparation of 111—V Compounds, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962). [16] L.T.P. Allen, ER. Weber, J. Washburn, Y.C. Pao and AG. Elliot, J. Crystal Growth 87 (1988) 193. [17] B.I. Miller, E.F. Schubert, U. Koren, A. Ourmazd, A.H. Dayem and R.J. Capik, AppI. Phys. Letters 49 (1986)
References [1] T.P. Pearsall and L.C.R. Thomson, Solid State Electron. 21(1978) 297.
1384.
o~o,
CRYSTAL OROWT H
OMCVD growth of InP, InGaAs, and InGaAsP on (110) InP substrates R. Bhat, M.A. Koza, D.M. Hwang, M.J.S.P. Brasil, R.E. Nahory Bellcore, 331 Newman Springs Road, Red Bank, New Jersey 07701-7040, USA
and K.Oe NTT Optoelectronics Laboratory, Atsugi 243-01, Japan
Device quality InP and InGaAsP (Ag = 1.0—1.55 ~sm) have been grown on 3~off (110) towards (111)B InP substrates. Shallow ledges running along the [1101direction form when the layer thickness is > 1 ~sm, indicating that a higher degree of misorientation is desirable. InGaAs showed severe facetting, even for thin layers, when grown on (110) substrates misoriented towards (111)B and a granular feature when grown on those misoriented towards (111)A. The InGaAsP layers have room temperature photoluminescence characteristics which are comparable to or better than those grown on (100) substrates. In addition, InGaAsP quantum wells, with compressive and tensile strain, have room temperature photoluminescence intensities which are 2—5 x higher than those 2 was obtained for a 3 quantum obtained (100) substrates. well laseron with a cavity lengthInofpreliminary 1.2 mm. experiments, a threshold current density of 1.5 kA/cm
1. Introduction
on (100) oriented substrates. Recently, there has
Epitaxial crystal growth studies have concentrated on the growth on (100) oriented substrates due to the fact that the natural cleavage planes are normal to the (100) crystal face and to each other, the ease with which epitaxial layers with excellent surface morphology can be grown, and the high quality of the devices fabricated with these layers. Epitaxial layers on substrates of other orientations, such as (110), can be advantageous for making use of the anisotropic properties of Ill—V compound crystals. The benefits of fabricating avalanche diodes [1] and optical modulators [2,3] in (110) GaAs or InP have been demonstrated. Quantum wells and superlattices have been intensively investigated in the past decade, due to their many unique physical properties, and potential application to novel devices. Again, most of these investigations have concentrated on growth
been increasing interest in the properties of quantum wells grown on substrates of unconventional orientations such as (110) [41and (111) [5,61,because both the energies and the oscillator strengths of the interband optical transitions are sensitive to the crystallographic direction of the epitaxial growth. In particular, studies on (110) oriented quantum wells are interesting, since the lower symmetry of the quantum confinement resuits in in-plane polarization anisotropy of the optical transition [4]. Liquid phase epitaxy (LPE) [2],molecular beam epitaxy (MBE) [3—7],and gas-source molecular beam epitaxy (GSMBE) [81 have been used for the successful growth of specular, device quality layers on (110) substrates. There are a few reports of GaAs growth by organometallic chemical vapor deposition (OMCVD) on (110); however, good morphology was only reported by Okamoto et al. [12],although some facetting is seen in their
0022-0248/92/$05.00 © 1992
—
Elsevier Science Publishers B.V. All rights reserved
312
R. Bhat et al.
/
OMCVD growth of InP, InGaAs, and !nGaAsP on (110) lnP substrates
2. Experimental procedure
Fig. 1. Typical morphology of an InP epitaxial layer on an exact (110) InP substrate. Marker represents 50 ~sm.
The growths were carried out in a horizontal, low pressure (76 Torr) OMCVD reactor using trimethyl alkyis of gallium (TMG) and indium (TM!), arsine, phosphine, and hydrogen as a carrier gas. n-Type and p-type doping was accomplished using hydrogen sulfide and diethyizinc, respectively. The substrates were tin (n 1 x 1018 cm3) or sulfur (n 5 x 1018 cm3) doped InP having a orientation of either (110) or 3°off (110) towards (11 1)A/B (hitherto designated as (1 10)A and (110)B, respectively). The misorientation direction was chosen based on the work of Allen et al. [7] for MBE of GaAs and that of Zamkotsian et al. [8] for GSMBE of InP based materials. 3. Results
layers. Furthermore, we are not aware of any
report of (110) quantum wells grown by OMCVD. In addition, there appears to be no report of any growth on (110) InP by OMCVD, except for that of GaAs 05Sb05 [13],where poor morphology was obtained. In this work, we report on the growth of InP, InGaAs, and InGaAsP on (110) and 3°off (110) towards (111)A/B InP substrates by OMCVD.
3.1. InP growth
InP growth was studied by varying the temperature in the range 550—675°C, the phosphine mole fraction from 4.2 x iO~to 3.3 x 10-2 and the TM! mole fraction from 1.7 X iO~to 8.75 X iO~.The layers on exact (110) substrates were highly facetted. A typical morphology is shown in
Fig. 2. Typical morphology of a 5 ~smthick InP layer on (a) a (110)B InP substrate showing the shallow ledges running along [110] and (b) a (110)A InP substrate showing the granular feature. Markers represent 10 ~sm.
R. Bhat eta!.
/ OMCVD growth of InP,
InGaA.~,and InGa.4sP on (110) InP substrates
fig. 1. At the rounded edges of the (110) substrates corresponding to tilting of the surface towards either the (111)A or B directions the facetting disappeared for some growth conditions. Consistent with this observation, highly specular, defect free surfaces were obtained on (110)A/B in the temperature range 550—575°C and TM! and phosphine mole fractions of 8.75 x iO~and ~ 1.6 x 10—2, respectively, resulting in a growth rate of 1.6 nm/s. A very fine granular feature was barely observable at high magnification (1000 x) under a Nomarski interference contrast microscope for (110)A but not (110)B substrates. For thick (— 5 ~m) layers, very shallow ledges running along the [110] direction appear on (110)B substrates (fig. 2a). This suggests that a misorientation an ~legreater than 3°would lead to a better morphology. For layers on (110)A substrates, the granular feature becomes better defined (fig. 2b). The background net donor concentration in these films was 3.5 x 1014 and 1 X 1015 cm3 on (110)A and (110)B substrates, re‘~
-
_______________________________
lOIS
1018
•.(100) x.(l1O)B (110) A
-
575°C TMI.8.75x105 PH 3. ~
1017
.
10~ x /
i0~~
z
10 0
/
.
____________________________ 2 4 6 8 10 12
spectively, as obtained by C—V measurements. Layers grown under the same conditions on (100)InP had a background net donor concentration of 3 x 1014 cm3. Hall measurements were not done since semi-insulating substrates were not available. A comparison of n-type doping, using hydrogen sulfide, was made by simultaneous growth on (100) and (1 10)A/B substrates. It can be seen in fig. 3 that n-doping is as much as 14 x more efficient on (110)B and 80 x less efficient on (110)A as compared to that on (100). The significantly higher incorporation of the n-type dopant on (110)B as compared to that on (110)A is consistent with the earlier work of Bhat et a. [14]. A similar comparison was also made of p-type doping using diethylzinc. The results are shown in fig. 4. It can be seen that Zn doping is as much as 4.5 x and 6 x times more efficient on (110)B and (110)A, respectively, as compared to that on (100). The higher acceptor incorporation efficiency on (110)A as compared to that on (110)B is again consistent with the earlier work of Bhat et al. [14]. 32 I GaAs
—
1O’~
40
H2S MOLE FRACTION — Fig. 3. Dependence of net donor concentration on H2S mole fraction for InP epitaxial InPlayers substrates. on (100), (110)A, and (110)B
313
n
dl Ga.AP an
n
th
s grow
As in the case of InP, the optimum growth temperature for InGaAs was found to be in the range 500—575°C. High mole fractions of TMI, TMG, and arsine were again used to obtain the best morphology and were 8.75 x iO~ 9.5 x iO~,and 8.3 x iO~,respectively, resulting in a growth rate of 3.0 nm/s. Layers grown on exact (110) substrates were highly facetted. However, unlike in the case of !nP, layers (<1 p~m thick) grown on (1 10)B showed facetting with ledges running along [110].The layers on (110)A were smoother with a slight graininess being barely visible under Nomarski interference contrast at high magnification (1000 x). These features become enhanced upon the growth of thick (5 ~m) layers as shown in figs. 5a and 5b. This again suggests that the misorientation angle is insufficient. The background net donor concentrations in these 3 films for (110)A were (2—3) and (110)B, x 10’~and respec(1 1.5) x i0’~cm tively.
314
R. Bhat eta!.
/
OMCVD growth of JnP, !nGaAs, and InGaAsP on (110) lnP substrates Table 1
1020 • .
(100)
x
(110) A
.
TMI -8.75 x iO~
-(110) B
PH
3 -1.6 x
io-~
2
8
~
InP substrates Bandgap wavelength (/Lm)
Linewidth (meV) (100) (110)
Intensity (100)
(110)
1.0 1.1
40.6 54
49.5 40.9
500 198
250 200
1.55
56.4
55.4
160
200
are nearly the same on both (110)A/B. Since the
10~~
/ / I l0’7~
Room temperature photoluminescence intensity (arbitrary units) and linewidths of InGaAsP alloys on (110)B and (100)
~
h
~
~
DEZ MOLE FRACTION —
Fig. 4. Dependence of net acceptor concentration on DEZ mole fraction for InP epitaxial layers on (100), (110)A, and (110)B InP substrates.
reactor was not optimized for uniform growth at 575°C, it is not possible to say if there is a systematic difference in the compositions obtamed on (110)A/B. However, specular morphologies, as seen using Nomarski interference contrast microscopy, were obtained only for (1 10)B, while layers on (1 10)A showed a granular surface (although it appeared specular to the naked eye). Depending on the thickness of the .
epitaxial layers, ledges running along [1101were occasionally seen, as in the case of InP. The
A range of InGaAsP compositions correspond-
layers were characterized using room tempera-
ing to bandgap wavelengths ranging from 1.0 to
ture (RT) photoluminescence (PL). In table 1 a
1.55 j~mhave been grown, lattice matched to InP, under the conditions optimized for InP and InGaAs, at 575°C.The compositions of the layers
comparison is made between the PL intensities and linewidths obtained for layers on (110)B and
(100). The layers on (100) were grown at 625°C
Fig. 5. Typical morphology of 5 ~m thick InGaAs epitaxiai layers on (a) (110)A and (b) (110)B InP substrates. Markers represent 10 ~sm.
R. Bhat eta!.
/
OMCVD growth of InP, !nGaAs, and InGaAsP on (110) InP substrates
and at approximately half the growth rate on (110), which are our optimized growth conditions for (100). It is seen that layers on (110)B are superior, with the exception of the Ag = 1.0 ~m quaternary, to those on (100). Considering the much larger data base we have on quaternaries on (100) it can be concluded that the RTPL characteristics of quaternaries on (110)B are cornparable to or better than those on (100). 3.3. InGaAs and InGaAsP quantum wells
The optimized growth conditions on (110)B substrates were used to grow single quantum wells (QWs) of InGaAs with nominal thicknesses of 2.5, 6.0, and 12.0 nm in one growth step. A thick (—~0.4 kin) reference !nGaAs layer was also grown. The PL spectrum at 5 K is shown in fig. 6. The figure shows 4 peaks corresponding to recombination in the thick InGaAs layer and in each of the 3 QW layers. It can be seen that the linewidths are comparable to those reported for (100) (17). Cross-section transmission microscopy (TEM) examination of these QWs shows (Fig. 7) that, whereas the InP layers are highly planar, the InGaAs surface breaks up into (110) treads and (111) risers, as may be expected on a (110) substrate misoriented by 3°towards the (111)B. This causes a fluctuation in the well thickness. The risers are only 2.3 nm high even for the 0.4 km thick reference layer. It should be noted here ‘—
315
that PL linewidths alone are not sufficient to judge the quality of QWs. The tendency for the InGaAs layers to facet and the planar growth front seen for !nP in fig. 7 are consistent with our observations on thick InP and InGaAs layer surface morphologies. In fig. 7 it can also be seen that the growth of InP on the facetted InGaAs rapidly planarizes the surface. Since the surface morphologies of quaternary layers (Ag = 1.0—1.55 p~m)were smooth and planar, several InGaAsP multiple QW layer structures with InGaAsP (Ag = 1.3 p~mfor compressive and 1.2 ~.amfor tensile strained wells) barrier layers were grown with a view towards 1.55 ~sm laser fabrication. The OW layers were 6 nm thick In062Ga038As086P014 (4a/a = + 0.17%), 2.4 nm thick In0 73Ga027As096P004 (6a/a = 14 nm thick In039Ga061As086P014 (ôa/a = 1.5%). Indeed, as anticipated, crosssection TEM examination of the compressive strained QWs indicated that the layers were planar. However, the tensile strained QWs were facetted as in the case of InGaAs QWs. Therefore, it appears that not only the As/P ratio but also the In/Ga ratio influences the formation of facets. The PL intensity from the compressive strained QWs was found to be 2—5 x higher than that for the best quantum wells grown by us on (100). + 0.94%), or —
‘~
4. Discussion 5K
1525.Onm
I
I
-
~E-6meV
1368~Onm 1254.0 nm ~7N.~-18 1200.00
~
i~so.onm
~E-9.3maY
1400.00 Wavelength, nm
~E-9.1maY 1600.00
Fig. 6. 5 K PL spectrum of InGaAs single quantum wells with nominal thicknesses of 2.5, 6.0, and 12 nm. The peak at 1525 nm is from the 0.4 ~im thick InGaAs reference layer.
The (110) orientation has traditionally been viewed as a difficult one for epitaxial growth because nucleation is difficult [151. Atoms on {110} surfaces form planar zigzag chains, each atom being bonded to two adjacent atoms in the chain. Each surface atom has one dangling bond. Therefore, the first atom depositing to start a new layer can make only one bond to the existing surface and three dangling bonds are created, an increase of two dangling bonds. Hence there is a potential barrier for nucleation. Once the first atom is in place however nucleation can proceed by the addition of atoms of the opposite type alternately at each end of the bonding chain without further nucleation barriers. The probabil-
316
R. Bhat eta!.
/
OMCVD growth of InP, InGaAs, and InGaAsP on (110) InP substrates
-~
Fig. 7. Cross-section TEM micrograph of InGaAs single quantum wells on a (110)B InP substrate. The dark bands are the InP barrier layers. The polycrystalline layer on the surface is an inadvertently grown GaP layer. The InGaAs reference layer shows well-developed facets.
ity of Ill—V interaction is increased when both III and V concentrations are high on the surface, being proportional to the product of the concentrations. It can also be argued that the growth temperature should be low so as to enhance the sticking coefficient of the V element. However, it cannot be so low as to significantly lower the cracking efficiency of the V hydride. These arguments are consistent with our experimental resuits wherein high III and V reagent mole fractions and low growth temperatures are found to be necessary for good epitaxy on (110). Allen et al. [7,16]found that in MBE of GaAs smooth epitaxial layers were obtained only on (110)A. They argue that misorientations towards (111)A produces stable (111)A ledges upon which As can nucleate and growth can proceed in a layer-by-layer mode. Zamkotsian et al. [8] were
able to obtain specular InP and InGaAsP layers on both (110)A and B using GSMBE, although it is not clear if one orientation is better than the other. In our work, it was found that misorientations towards (11 1)B gave superior layers of !nP and InGasAsP but InGaAs grew better on substrates misoriented towards (111)A. However, the surface morphoiogies of thick (5 ~sm) InP and InGaAs layers on (110)B suggest that a higher degree of misorientation towards (111)B may produce more planar InP and InGaAs surfaces. 5. Lasers A separate confinement heterostructure laser structure, with the active layer consisting of three 6 nm thick !n062Ga038As096P0~(Ag = 1.64 /Lm)
R. Bhat et aL
/ OMCVD growth of InP, InGaAs,
quantum wells and 20 nm thick InGaAsP (Ag = 1.3 ~sm) barriers was processed into 50 ~m wide oxide stripe broad area lasers. The threshold current density for lasers with a cavity length of 1.2 mm was 1.5 kA/cm2, which is comparable to the GSMBE results of Zamkotsian et al. [8].With further optimization of the growth and laser structures and utilization of compressive and tensue strain in the quantum wells, substantial reduction in the threshold current density is anticipated. 6. Conclusions The OMCVD growth of device quality InP based materials on (1 10)B substrates has been achieved. Further improvements in the surface morphology, particularly of !nGaAs and of thick !nP, may be expected for layers grown on substrates with larger misorientations. The doping behaviour of sulfur and zinc in (110) InP has been established. It is found that the room temperature photoluminescence of InGaAsP grown on (110)B is comparable to or better than those on (100). Similarly, quantum wells on (110) have a higher PL intensity than those on (100). Finally, preliminary results indicate that quantum well lasers on (110)B are promising.
and InGaAsP on (110) InP substrates
317
[2] J. McKenna and F.K. Reinhart, J. Appl. Phys. 47 (1976) 2069. [3] F. Zamkotsian, C. Artigue, J-L. Peyre, M. Lambert, D. Bonnevie and J. Benoit, Electron. Letters 26 (1990) 1560. [4] D. Gershoni, I. Brener, GA. Baraff, S.N.G. Chu, L.N. Pfeiffer and K. West, Phys. Rev B 44 (1991) 1930. [5] T. Hayakawa, M. Kondo, T. Suyama, K. Takahashi, S. Yamamoto and T. Hijikata, Japan. J. AppI. Phys. 26 (1987)L302. [6] L.W. Molenkamp, R. Eppenga, G.W. ‘t Hooft, P. Dawson, C.T. Foxon and KJ. Moore, Phys. Rev. B 38 (1988) 4314. [7] L.T.P. Allen, ER. Weber, J. Washburn and Y.C. Pao, Appl. Phys. Letters 51(1987) 670. [8] F. Zamkotsian, F. Poingt, L. Le Gouezigou, F. Gaborit, J.-G. Provost, C. Artigue and J. Benoit, in: Proc. Conf. on InP and Related Materials, 1991, p. 584. [9] V. Gottschalck, W.-H. Petzke and E. Butter, Kristall Tech. 9 (1974) 209. [10] D.H. Reep and S.K. Ghandhi, J. Crystal Growth 61 (1983) 449. [11] K. Kamon, M. Shimazu, K. Kimura, M. Mihara and M. Ishi, J. Crystal Growth 84 (1987) 126. [12] K. Okamoto, M. Furuta and K. Yamaguchi, Japan. J. Appl. Phys. 27 (1988) L2121. [13] H.R. Jen, M.J. Jou, Y.T. Cherng and G.B. Stringfellow, J. Crystal Growth 85 (1987) 175. [14] R. Bhat, C. Caneau, C.E. Zah, M.A. Koza, W.A. Bonner, P.M. Hwang, S.A. Schwarz, S.G. Menocal and F.G. Favire, J. Crystal Growth 107 (1991) 772. [15] R.C. Sangster, in: Compound Semiconductors, Vol. 1, Preparation of 111—V Compounds, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962). [16] L.T.P. Allen, ER. Weber, J. Washburn, Y.C. Pao and AG. Elliot, J. Crystal Growth 87 (1988) 193. [17] B.I. Miller, E.F. Schubert, U. Koren, A. Ourmazd, A.H. Dayem and R.J. Capik, AppI. Phys. Letters 49 (1986)
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1384.