- Email: [email protected]
HBT laser driver
Journal of Crystal Growth 93 (1988) 885-891 North-Holland, Amsterdam
885
MOVPE STUDIES FOR A MONOLITHICALLY INTEGRATED DH L A S E R / H B T LASER DRIVER P. SPEIER, U. KOERNER, A. NOWITZKI, F. GROTJAHN, F.J. T E G U D E and K. WONSTEL Standard Elektrik Lorenz AG, Research Centre, Optoelectronic Components Division, Lorenzstrasse 10, D-7000 Stuttgart 40, Fed. Rep. of Germany
The two basic tools for the monolithic integration of optoelectronic and electronic devices are described: Firstly, the growth of semi-insulating InP layers with ultra high resistivities of up to p = 2 × l09 ,Q cm. Secondly, the selective area growth resulting in planar surfaces. Undoped InP and lnGaAs layers with background doping levels c~f around n =5×1014 cm -3 and electron mobilities of P'300/77K = 5000/150.000 cm2//V-s and /3.300/77K =11500/100000 cm2/V.s are achieved. Doping characteristics, especially for the first time manganese doping by MOVPE, are reported. Discrete double-heterostructure bipolar transistors have been fabricated with the highest current gain of fl = 26000 reported to date for this material system. I n G a A s P / I n P BH lasers with semi-insulating lnP current-blocking layers and a planar surface have been realized. Threshold currents of 26 mA and extremely low capacitances of 3-5 pF promising excellent high frequency modulation characteristics have been achieved.
1. Introduction
2. Experimental set-up
In recent years research and development on long wavelength (X = 1.3-1.65 ,urn) optoelectronics have rapidly expanded. Excellent results have been reported for discrete lasers and detectors [1,2]. Monolithic integration of these devices is considered to be an approach towards good highspeed performance, highly reliable and small size systems and low cost fabrication. However, only few attempts such as integration of InGaAsP lasers with either InP metal-insulator-semiconductor field-effect transistors (MIS-FET) [3] or I n P / I n G a A s P / I n P double-heterostructure bipolar transistors (DHBT) [4] have been reported. ~n this paper we report on MOVPE studies for an I n G a A s P / I n P DH laser integrated with l n P / I n G a A s / I n P DHBTs. Growth results on undoped and n-type or p-type doped InP and InGaAs layers are described. Two basic tools for the monolithic integration of optoelectronic devices are presented: the growth of semi-insulating InP layers combined with a selective area growth resuiting in planar surfaces. Device results for discrete lasers with semi-insulating current-blocking layers and for DIABTs using these techniques are reported.
The MOVPE growth system consists of a gas handling system for 10 metalorganic and 5 hydride sources with a vent/reactor fast switching capability for each line and a horizontal, RFheated cold wall reactor with rectangular crosssection and a SiC coated graphite susceptor. The system is operated both at atmospheric and at reduced pressure. Trimethylindium (TMIn) and trimethylgallium (TMGa) are used for column Ill sources, and 100% arsine (AsH 3) and 100% phosphine (PH 3) for column V sources. Dimethylcadmium (DMCd) and tricarbonyl(methylcyclopentadienyl)manganese (COTMn) are used for ptype doping, hydrogen sulphide (H2S) is used for n-type doping and bis(cyclopentadienyl)iron (ferrocene) (BCPFe) is used for growth of semiinsulating InP layers. Palladium diffused hydrogen or purified argon are the carrier gases in the system. The growth temperature ranges from 590°C to 650°C. The V / I I I ratio for lnP growth is typically 200 and 50 for InGaAs. The growth temperature in the indicated range as well as the reactor pressure do not have a significant influence on the carrier concentration of undoped layers. Thus for the growth of device structures with p-type doped regions a growth temperature
0022-0248/88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P. Speier et a L / Monolithically integrated DH laser/HBT laser driver
886
below To = 650 ° C is used to reduce outdiffusion of dopants. Typical growth rates for InP and InGaAs layers are 1.5 /~m/h and 2.3 /~m/h, respectively.
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3 1. Undoped layer characteristics
"~
The background carder concentration and mobilities for InP and InGaAs layers are shown in table 1. The background doping level depends on the quality and the cleaning process of the substrates, and even more on the purity of the source materials, e.g. TMIn. Double crystal diffractometer measurements show a full width at half maximum (FWHM) of 19 arcsec for lnGaAs layers indicating high crystal quality.
.
H2S: • TG:650°C • TG: 620"C lO15,
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Dopont Precursor Mole Froction Fig. 1. Sulphur doping in lnP layers for different growth temperatures at atmosphericpressure
3.2. Doping characteristics For n-type doping a 10% mixture of H2S in H 2 is used for both InP and lnGaAs. There is no significant difference in the doping characteristic between atmospheric [5] and reduced [6] pressure growth. In fig. 1 the n-type doping of InP is shown for different growth temperatures. A reduced doping efficiency for higher temperatures, as expected from ref. [7], can be seen. p-Type doping of InP using DMCd has been reported previously [8,9]. In fig. 2 the p-type doping of InGaAs is shown for a growth temperature of 620" C and a reactor pressure of 1 bar. At reduced pressure the doping efficiency of cadmium drastically decreases [9]. Thus an alternative p-type dopant is necessary. If outdiffusion is critical, zinc cannot be used [8]. Manganese seemed to be an interesting candidate in LPE growth of p-type InGaAs [10]. For the first time manganese doping in MOVPE growth is reported, Using COTMn Table 1 Background doping and cartier mobl|lty at 300 and 77 K
InP InGaAs
r/3~;K
/'t 300 K
r/77K
~'£77 K
(crn -3 )
(cm2/V. s)
(cm -3 )
(cm2/V.s~
5×1014 5 × 1014
5000 11 500
1 x 1 0 la 3 × 10 TM
150000 100000
excellent doping characteristics for InGaAs layers with ultra sharp doping profiles are achieved with a spatial abruptness of < 20 nm for one order decay, which is the resolution limit of the electrochemical profile plotter used. Unfortunately almost no vapor pressure data are available for
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Fig. 2. Cadmium doping in InGa.As layers at atmospheric pressure
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Fig. 3. Manganese and sulphur doping in InGaAs layers at reduced pressure.
COTMn. Therefore, in fig. 3 the p-type doping level is shown as a function of the carrier gas flow through the C O T M n bubbler. In addition, the sulphur doping in InGaAs for a growth temperature of Tc = 5 9 0 ° C and a reactor pressure of 50 mbar is shown in fig. 3. In fig. 4 the electron hall mobility is shown as a function of the net carrier concentration for lattice matched InGaAs layers. No memory effect in the reactor can be observed using the dopants described above.
While in OaAs ;.he intrinsic deep level EL2 allows the growth of undoped semi-inm:lating material, in InP no equivaient intrinsic defect is known. Thus doping with elements causing deep levels is necessary. Encouraging results for MOVPE grown semi-insulating lnP layers doped with iron have been reported [11,12]. Using ferrocene as metalorgarJc source material we have grown semi-insulating InP layers with ultra high resistivities of up to p = 2 × 109 12 cm at 300 K (fig. 5). The g,'owth conditions [12] and the electrical characterization [12,13] have been described previously. The resistivity of the layers depends on the iron content in the gas phase and can be varied from p = 2 × 1 0 .7 i cm to p = 2 x 1 0 9 I2 cm. Using fig. 5 the activation energy can be determined to 0.66 eV from the conduction band which is in excellent agreement with results for the Fe z+ state from other authors [14,15].
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Carrier Concentration(No-NA) [crn"3] Fig. 4. Electron hall mobility as a function of the net carrier concentration ( N D - NA ) for lattice matched InGaAs layers.
c~o cbo 3~o 3bo T [K3
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Fig. 5. Resistivity of a semi-insulating InP layer ageinst re cipro~al temperature.
P. Speier et at / Monolithically integrated DH laser/HBT laser driver
888
4.2. Selective area growth i
Selective MOVPE in principle allows for an enhanced flexibility for the design of optoelectronic circuits. However, difficult pre-studies on the growth mechanism of structured and partially masked InP substrates have been necessary. After a cleaning step, lnP substrates are coated with a dielectric layer of CVD SiO 2. The patterning is done by a standard photolithography using three types of masks (fig. 6). The SiO 2 windows are etched with buffered HF solution. Various semiconductor etchants are used to reveal different crystallographic orientations [16]. Regrowth is done at atmospheric and reduced pressure under the growth conditions described above. At atmospheric pressure polycrystalline growth on top of the SiO2 mask occurs [17-19]. By application of SiO 2 stripes with a stripe width less than 20 #m, polycrystalline growth can be effectively suppressed [19]. At reduced pressure ( < 100 mbar) no
Fig. 6. The window patterns of the SiO2 masks used. Markers represent: (a) 500 ~m; (b) 100/am; (c) 500 #m.
Fig. 7. Cross-section of a structured and SiO2-masked inP substrate with strong edge overgorwth regrown at atmospheric pressure.
polycrystalline growth is found even for geometries broader than 500/.tm. This demonstrates that the surface mobility of the species is strongly increased by reducing the reactor pressure. A strong edge-overgrowth along the mask boundary (fig. 7) is frequently observed for atmospheric and even for reduced pressure MOVPE [17-20]. But using appropriate growth conditions and an etchant causing a long SiO: overhang [19,21], selective growth can be done resulting in planar surfaces with no edge-overgrowth (fig. 8) even at atmospheric pressure. At reduced pressure there is one more degree of freedom due to the enhanced surface mobility of
Fig. 8. Cross-section of a structured and SiO2-masked [nP substrate without edge overgrowth regrown at atmospheric pressure.
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progress is shown in a schematic draft by dashed lines. Depending on the application the growth can be interrupted resulting again in almost planar surfaces.
8
5. Application to devices Up to now the MOVPE techniques described above are used to grow layer structures for discrete devices. Double-heterostructure bipolar transistors (DHBTs) are realized and LPE grown InG a A s P / I n P heterostructure lasers are buried with semi-insulating InP current-blocking layers, both by atmospheric pressure MOVPE. 0"
Fig. 9. Cross-section of a dovetail structure with SiO2 on top regrown at reduced pressure: (a) schematic draft with growth lines to demonstrate growth progress; (b)+(c) viewgraphs of regrown structures with different layer thicknesses for the selectively grown layers. Markers represent: (b) 4 /xm; (c) 8/~m.
5.1. InP / In GaAs / InP double-heterostructure-bipolar-transistors A seven-layer structure as shown in fig. 10 is grown at T6 = 650 ° C on SI lnP substrates using H2S for n-type doping and DMCd for p-type doping. The doping level and layer thickness are measured by an electrochemical profile plotter (Polaron) for the complete structure. The highly n-doped subcollector layer reduces the collector access resistance and thus the on-resistance of the device. The undoped InGaAs spacer layers sandwiching the p-InGaAs base layer reduce the influence of the conduction band spike at the heterointerface thereby enhancing the emitter and collector efficiency and the high speed performance. A detailed description of the device processing and characterization will be published elsewhere [22]. A current gain of fl - 26 000 is the InGaAs:S
n > 5 x1018 crn -3
d=0 2htm
InP: S
n = 1 x 10~7 cm "3
d : 0 4Nrn
undoped
d<0.2~m
InGoAS
"'lnGaAs:Cd
the species. Therefore also a dovetail geometry which might be favourable for laser applications due to a better ratio of c o n t ~ t to active region width, can be regrown wiO.o,Jt edge-overgrowth. In figs. 9b and 9c, viewgraphs of a cleaved crosssection of a dovetail structure with Si02 on top regrown with InP is shown. In fig. 9a the growth
p.-?~1017cm '-3 '" -d=0.2htrn
InGaAs
undopcd
d
InP:S
n = 1 x 10~7crn "3
d=0.5.~tm
InP:S
n= 5 x l O l B c m "3
d=0.t, Atm
InP: F¢ substrat¢ Fig. 10. Seven-layer structure for a DHBT.
890
P. Speier et al. / Monolithically integrated DH laser/ HBT laser driver InOaAaP
p-lnG~P
aemi-in',ulatlng InP
i~-InP
n-lnP
]
Fig. 11. Cross-section of a planar-embedded DH laser with semi-insulatingInPcurrent-blockinglayers. highest reported to date for this material system [23]. Output currents up to I = 100 mA for devices with a 4 x 10 -5 cm 2 emitter-base area me obtained which are high enough for application as laser driver. 5.2. DH-laser with semi-insulating current-blocking layers
Using the techniques described abov~,, a DH laser is fabricated: in a first step, standard fourlayer epitaxy is performed on an n-InP substrate by LPE, comprising an n-InP buffer layer, an InGaAsP active layer (k = 1.3 /tm), a p-lnP embedding layer and a p-InGaAsP contact layer. In a second step. the laser stripe is defined by etching a 4 ~tm high and 3 ~ m wide mesa with SiO 2 as the etch mask. The SiO 2 is also used as a self-aligned mask for the selective growth of semi-insulating inP at both sides of the laser rib. The structure resembles that shown in fig. 8. A cross-section of the structure is schematically shown in fig. 11. Even with this not yet optimized structure for laser application, due to the wide active region and the small contact stripe, threshold currents of -/th = 26 mA and an output power of 12 mW at 100 mA are achieved under pulsed operation. Thus our experiments indicate effective suppression of leakage currents and low recombination at the interface between active layer and Fe-doped InP. Excellent ~gh-frequency modulation characteristics are e,~pected due to a very low capacitance value of 3-5 pF measured for these lasers, which is one order of magnitude lower than for conventional BH lasers [1]. The dovetail geometry discussed above wi!l bring additional improvements in terms of threshold current and contact resistance.
6. Conclusion We have demonstrated MOVPE growth both at atmospheric and at reduced pressure of high quality InP and InGaAs layers and the doping characteristics for these materials. For the first time manganese doping is reported for MOVPE growth. We have developed the basic tools for monolithic integration of optoelectronic devices such as selective MOVPE resulting in planar surfaces and selective growth of semi-insulating InP layers with ultra high resistivities of p---2 × 10 9 Q cm. Discrete double-heterostructure bipolar transistors have been fabricated with the highest current gain of fl = 26000 reported to date for this material system. I n G a A s P / I n P lasers with semi-insulating InP current blocking layers and a planar surface have been realized. Threshold currents of 26 mA and extremely low capacitance values of 3-5 pF have been achieved.
Acknowledgements We wish to thank our colleagues C. Emele, A. Ifl~inder, B. Knapp, M. Klenk, W. Kuebart, G. Miiller, M. Schilling, I. SchiSttle, H. Schweizer, A. Stumpf, P. Wiedemann and O. Hildebrand for technical support and helpful discussions. Financal support by the European Strategic Programme for Research and Development in Information Technology (ESPRIT Project 263 B) and by the German Ministry of Research and Technology (BMFT) is gratefully acknowledged.
References [1] W. Streifer, Ed., Special issue on semiconductorlasers, .I. Quantum Electron. QE-23 (1987). [2] H. Haupt and O. Hildebrand, IEEE J. Selected Areas Commun. SAC 4 (1986) 444. [3] U. Koren, S. Margalit, T.R Chen, K. Yu, A. Yariv, N. Bar-Chaim, K.Y. Lan and I. Ury, IEEE J. Quantum Electron. QE-1 (1982) 1653. [4] J. Shibata, I. Nakao, Y. Sasai, S. Kimura, N. Hase and H. Serizawa, Appl. Phys. Letters 451 (1984) 191. [5] A.W. Nelson, R.H. Moss, P.C. Spurdens, S. Cole and S. Wong, Brit. TelecomTechnol. J. 4 No. 2 (1986) 85. [6] M. Razeghi and J.P. Duchemin, J. Crystal Growth 64 (1983) 76.
P. Speier et a L / Monolithically integrated DH laser / HBT laser driver [7] G,B. Stringfellow, J. Crystal Growth 75 (1986) 91. [8] A.W. Nelson and L.D. Westbrook, J. Crystal Growth 68 (1984) 102. [9] C. Blaauw, B. Emmerstorfer and A.J. SpringThorpe, J. Crystal Growth 84 (1987) 431. [10] M. Schilfing, W. Kuebart, G. Miiller, G. Schemmel and F.J. Tegude, in: Proc. 16th European Solide State Device Research Conf., 1986, p. 129. [11] J. Long, V. Riggs and W. Johnston, J. Crystal Growth 69 (1984) 10. [12] P. Speier, G. Schemmel and W. Kuebart, Electron. Letters 22 (1986) 1216. [13] A. Macrander, J. Long, V. Riggs, A. Bloemeke and W. Johnston, Appl. Phys. Letters 45 (1984) 1297. [14] G. Iseler, in: Proc. 7th Intern. Syrup. on GaAs and Related Compounds, St. Louis, MO, 1978, Inst. Phys. Conf. Ser. 45, Ed. C.M. Wolfe (Inst. Phys., London-Bristol, 1979) p. 144.
89i
[15] A. Juhl and D. Bimberg, in: Pro¢. 5th Conf. on Semi-Insulating I I I - V Materials, 1986, p. 477. [16] S. Adachi and H. Kawaguchi, J. Electrochem. Soc. 128 (1981) 1342. [17] P. Speier, A. Nowitzki and G. Schemmel, presentation at the 2nd MOVPE Workshop, Cornell University, 1985. [18] C. Blaauw, A. Szaphlonczay, K. Fox and B. Emmerstorfer, J. Crystal Growth 77 (1986) 326. [19] P. Speier, K. Wiinstel and F.J. Tegude, Electron. Letters 23 (1987) 1363. [20] A, Clawson, C. Hanson and T. Vu, J. Crystal Growth 77 (1986) 334. [21] T. Sanada, K. Nakai, K. Wakao, M, Knmo and S. Yamakoshi, Appl. Phys. Letters 51 (1987) 1054. [22] F,J. Tegude et al., to be published. [23] J.R. Hayes, R. Bhat, H. Schumacher and M. Koza, Electron. Letters 23 (1987) 1298.
885
MOVPE STUDIES FOR A MONOLITHICALLY INTEGRATED DH L A S E R / H B T LASER DRIVER P. SPEIER, U. KOERNER, A. NOWITZKI, F. GROTJAHN, F.J. T E G U D E and K. WONSTEL Standard Elektrik Lorenz AG, Research Centre, Optoelectronic Components Division, Lorenzstrasse 10, D-7000 Stuttgart 40, Fed. Rep. of Germany
The two basic tools for the monolithic integration of optoelectronic and electronic devices are described: Firstly, the growth of semi-insulating InP layers with ultra high resistivities of up to p = 2 × l09 ,Q cm. Secondly, the selective area growth resulting in planar surfaces. Undoped InP and lnGaAs layers with background doping levels c~f around n =5×1014 cm -3 and electron mobilities of P'300/77K = 5000/150.000 cm2//V-s and /3.300/77K =11500/100000 cm2/V.s are achieved. Doping characteristics, especially for the first time manganese doping by MOVPE, are reported. Discrete double-heterostructure bipolar transistors have been fabricated with the highest current gain of fl = 26000 reported to date for this material system. I n G a A s P / I n P BH lasers with semi-insulating lnP current-blocking layers and a planar surface have been realized. Threshold currents of 26 mA and extremely low capacitances of 3-5 pF promising excellent high frequency modulation characteristics have been achieved.
1. Introduction
2. Experimental set-up
In recent years research and development on long wavelength (X = 1.3-1.65 ,urn) optoelectronics have rapidly expanded. Excellent results have been reported for discrete lasers and detectors [1,2]. Monolithic integration of these devices is considered to be an approach towards good highspeed performance, highly reliable and small size systems and low cost fabrication. However, only few attempts such as integration of InGaAsP lasers with either InP metal-insulator-semiconductor field-effect transistors (MIS-FET) [3] or I n P / I n G a A s P / I n P double-heterostructure bipolar transistors (DHBT) [4] have been reported. ~n this paper we report on MOVPE studies for an I n G a A s P / I n P DH laser integrated with l n P / I n G a A s / I n P DHBTs. Growth results on undoped and n-type or p-type doped InP and InGaAs layers are described. Two basic tools for the monolithic integration of optoelectronic devices are presented: the growth of semi-insulating InP layers combined with a selective area growth resuiting in planar surfaces. Device results for discrete lasers with semi-insulating current-blocking layers and for DIABTs using these techniques are reported.
The MOVPE growth system consists of a gas handling system for 10 metalorganic and 5 hydride sources with a vent/reactor fast switching capability for each line and a horizontal, RFheated cold wall reactor with rectangular crosssection and a SiC coated graphite susceptor. The system is operated both at atmospheric and at reduced pressure. Trimethylindium (TMIn) and trimethylgallium (TMGa) are used for column Ill sources, and 100% arsine (AsH 3) and 100% phosphine (PH 3) for column V sources. Dimethylcadmium (DMCd) and tricarbonyl(methylcyclopentadienyl)manganese (COTMn) are used for ptype doping, hydrogen sulphide (H2S) is used for n-type doping and bis(cyclopentadienyl)iron (ferrocene) (BCPFe) is used for growth of semiinsulating InP layers. Palladium diffused hydrogen or purified argon are the carrier gases in the system. The growth temperature ranges from 590°C to 650°C. The V / I I I ratio for lnP growth is typically 200 and 50 for InGaAs. The growth temperature in the indicated range as well as the reactor pressure do not have a significant influence on the carrier concentration of undoped layers. Thus for the growth of device structures with p-type doped regions a growth temperature
0022-0248/88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P. Speier et a L / Monolithically integrated DH laser/HBT laser driver
886
below To = 650 ° C is used to reduce outdiffusion of dopants. Typical growth rates for InP and InGaAs layers are 1.5 /~m/h and 2.3 /~m/h, respectively.
1019.
x/ '~E 1018
3. Basic materials quality > lo17, ,,--I
3 1. Undoped layer characteristics
"~
The background carder concentration and mobilities for InP and InGaAs layers are shown in table 1. The background doping level depends on the quality and the cleaning process of the substrates, and even more on the purity of the source materials, e.g. TMIn. Double crystal diffractometer measurements show a full width at half maximum (FWHM) of 19 arcsec for lnGaAs layers indicating high crystal quality.
.
H2S: • TG:650°C • TG: 620"C lO15,
x TO: SS0°C ~ i|
10-8
•
•
i
i|
10-7
i
•
ii
I
10-6
i
i
i
i
I
10-5
Dopont Precursor Mole Froction Fig. 1. Sulphur doping in lnP layers for different growth temperatures at atmosphericpressure
3.2. Doping characteristics For n-type doping a 10% mixture of H2S in H 2 is used for both InP and lnGaAs. There is no significant difference in the doping characteristic between atmospheric [5] and reduced [6] pressure growth. In fig. 1 the n-type doping of InP is shown for different growth temperatures. A reduced doping efficiency for higher temperatures, as expected from ref. [7], can be seen. p-Type doping of InP using DMCd has been reported previously [8,9]. In fig. 2 the p-type doping of InGaAs is shown for a growth temperature of 620" C and a reactor pressure of 1 bar. At reduced pressure the doping efficiency of cadmium drastically decreases [9]. Thus an alternative p-type dopant is necessary. If outdiffusion is critical, zinc cannot be used [8]. Manganese seemed to be an interesting candidate in LPE growth of p-type InGaAs [10]. For the first time manganese doping in MOVPE growth is reported, Using COTMn Table 1 Background doping and cartier mobl|lty at 300 and 77 K
InP InGaAs
r/3~;K
/'t 300 K
r/77K
~'£77 K
(crn -3 )
(cm2/V. s)
(cm -3 )
(cm2/V.s~
5×1014 5 × 1014
5000 11 500
1 x 1 0 la 3 × 10 TM
150000 100000
excellent doping characteristics for InGaAs layers with ultra sharp doping profiles are achieved with a spatial abruptness of < 20 nm for one order decay, which is the resolution limit of the electrochemical profile plotter used. Unfortunately almost no vapor pressure data are available for
1018. ,-? E
j/
1017.
>~
÷/
DMCd TG=620°C
£3 1015. I ¢'1
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Fig. 2. Cadmium doping in InGa.As layers at atmospheric pressure
P. Speier et al. / Monolithically integrated DH laser/HBT laser driver
/
6,
/// =
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4. Basic too~s for integration
m*"
/
/
4. l. Growth of semi-insulating InP
/
"~ 101"/. •- J
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.C
6"
0
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,/
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/
4'
,,/
i#
o,oe' o,i'
012
887
, i m 1.0 , o.~ 0,6 0.e Gas Flow [sccm]
, 2,0
Fig. 3. Manganese and sulphur doping in InGaAs layers at reduced pressure.
COTMn. Therefore, in fig. 3 the p-type doping level is shown as a function of the carrier gas flow through the C O T M n bubbler. In addition, the sulphur doping in InGaAs for a growth temperature of Tc = 5 9 0 ° C and a reactor pressure of 50 mbar is shown in fig. 3. In fig. 4 the electron hall mobility is shown as a function of the net carrier concentration for lattice matched InGaAs layers. No memory effect in the reactor can be observed using the dopants described above.
While in OaAs ;.he intrinsic deep level EL2 allows the growth of undoped semi-inm:lating material, in InP no equivaient intrinsic defect is known. Thus doping with elements causing deep levels is necessary. Encouraging results for MOVPE grown semi-insulating lnP layers doped with iron have been reported [11,12]. Using ferrocene as metalorgarJc source material we have grown semi-insulating InP layers with ultra high resistivities of up to p = 2 × 109 12 cm at 300 K (fig. 5). The g,'owth conditions [12] and the electrical characterization [12,13] have been described previously. The resistivity of the layers depends on the iron content in the gas phase and can be varied from p = 2 × 1 0 .7 i cm to p = 2 x 1 0 9 I2 cm. Using fig. 5 the activation energy can be determined to 0.66 eV from the conduction band which is in excellent agreement with results for the Fe z+ state from other authors [14,15].
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Carrier Concentration(No-NA) [crn"3] Fig. 4. Electron hall mobility as a function of the net carrier concentration ( N D - NA ) for lattice matched InGaAs layers.
c~o cbo 3~o 3bo T [K3
2so
Fig. 5. Resistivity of a semi-insulating InP layer ageinst re cipro~al temperature.
P. Speier et at / Monolithically integrated DH laser/HBT laser driver
888
4.2. Selective area growth i
Selective MOVPE in principle allows for an enhanced flexibility for the design of optoelectronic circuits. However, difficult pre-studies on the growth mechanism of structured and partially masked InP substrates have been necessary. After a cleaning step, lnP substrates are coated with a dielectric layer of CVD SiO 2. The patterning is done by a standard photolithography using three types of masks (fig. 6). The SiO 2 windows are etched with buffered HF solution. Various semiconductor etchants are used to reveal different crystallographic orientations [16]. Regrowth is done at atmospheric and reduced pressure under the growth conditions described above. At atmospheric pressure polycrystalline growth on top of the SiO2 mask occurs [17-19]. By application of SiO 2 stripes with a stripe width less than 20 #m, polycrystalline growth can be effectively suppressed [19]. At reduced pressure ( < 100 mbar) no
Fig. 6. The window patterns of the SiO2 masks used. Markers represent: (a) 500 ~m; (b) 100/am; (c) 500 #m.
Fig. 7. Cross-section of a structured and SiO2-masked inP substrate with strong edge overgorwth regrown at atmospheric pressure.
polycrystalline growth is found even for geometries broader than 500/.tm. This demonstrates that the surface mobility of the species is strongly increased by reducing the reactor pressure. A strong edge-overgrowth along the mask boundary (fig. 7) is frequently observed for atmospheric and even for reduced pressure MOVPE [17-20]. But using appropriate growth conditions and an etchant causing a long SiO: overhang [19,21], selective growth can be done resulting in planar surfaces with no edge-overgrowth (fig. 8) even at atmospheric pressure. At reduced pressure there is one more degree of freedom due to the enhanced surface mobility of
Fig. 8. Cross-section of a structured and SiO2-masked [nP substrate without edge overgrowth regrown at atmospheric pressure.
P. Speier et a L / Monolithically integrated DH laser/ H B T laser driver ......
SiO2
....... _ - , \~ ..................... I".,. . . . . . . . .
II11111 IIII.LIJJI.I/
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progress is shown in a schematic draft by dashed lines. Depending on the application the growth can be interrupted resulting again in almost planar surfaces.
8
5. Application to devices Up to now the MOVPE techniques described above are used to grow layer structures for discrete devices. Double-heterostructure bipolar transistors (DHBTs) are realized and LPE grown InG a A s P / I n P heterostructure lasers are buried with semi-insulating InP current-blocking layers, both by atmospheric pressure MOVPE. 0"
Fig. 9. Cross-section of a dovetail structure with SiO2 on top regrown at reduced pressure: (a) schematic draft with growth lines to demonstrate growth progress; (b)+(c) viewgraphs of regrown structures with different layer thicknesses for the selectively grown layers. Markers represent: (b) 4 /xm; (c) 8/~m.
5.1. InP / In GaAs / InP double-heterostructure-bipolar-transistors A seven-layer structure as shown in fig. 10 is grown at T6 = 650 ° C on SI lnP substrates using H2S for n-type doping and DMCd for p-type doping. The doping level and layer thickness are measured by an electrochemical profile plotter (Polaron) for the complete structure. The highly n-doped subcollector layer reduces the collector access resistance and thus the on-resistance of the device. The undoped InGaAs spacer layers sandwiching the p-InGaAs base layer reduce the influence of the conduction band spike at the heterointerface thereby enhancing the emitter and collector efficiency and the high speed performance. A detailed description of the device processing and characterization will be published elsewhere [22]. A current gain of fl - 26 000 is the InGaAs:S
n > 5 x1018 crn -3
d=0 2htm
InP: S
n = 1 x 10~7 cm "3
d : 0 4Nrn
undoped
d<0.2~m
InGoAS
"'lnGaAs:Cd
the species. Therefore also a dovetail geometry which might be favourable for laser applications due to a better ratio of c o n t ~ t to active region width, can be regrown wiO.o,Jt edge-overgrowth. In figs. 9b and 9c, viewgraphs of a cleaved crosssection of a dovetail structure with Si02 on top regrown with InP is shown. In fig. 9a the growth
p.-?~1017cm '-3 '" -d=0.2htrn
InGaAs
undopcd
d
InP:S
n = 1 x 10~7crn "3
d=0.5.~tm
InP:S
n= 5 x l O l B c m "3
d=0.t, Atm
InP: F¢ substrat¢ Fig. 10. Seven-layer structure for a DHBT.
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P. Speier et al. / Monolithically integrated DH laser/ HBT laser driver InOaAaP
p-lnG~P
aemi-in',ulatlng InP
i~-InP
n-lnP
]
Fig. 11. Cross-section of a planar-embedded DH laser with semi-insulatingInPcurrent-blockinglayers. highest reported to date for this material system [23]. Output currents up to I = 100 mA for devices with a 4 x 10 -5 cm 2 emitter-base area me obtained which are high enough for application as laser driver. 5.2. DH-laser with semi-insulating current-blocking layers
Using the techniques described abov~,, a DH laser is fabricated: in a first step, standard fourlayer epitaxy is performed on an n-InP substrate by LPE, comprising an n-InP buffer layer, an InGaAsP active layer (k = 1.3 /tm), a p-lnP embedding layer and a p-InGaAsP contact layer. In a second step. the laser stripe is defined by etching a 4 ~tm high and 3 ~ m wide mesa with SiO 2 as the etch mask. The SiO 2 is also used as a self-aligned mask for the selective growth of semi-insulating inP at both sides of the laser rib. The structure resembles that shown in fig. 8. A cross-section of the structure is schematically shown in fig. 11. Even with this not yet optimized structure for laser application, due to the wide active region and the small contact stripe, threshold currents of -/th = 26 mA and an output power of 12 mW at 100 mA are achieved under pulsed operation. Thus our experiments indicate effective suppression of leakage currents and low recombination at the interface between active layer and Fe-doped InP. Excellent ~gh-frequency modulation characteristics are e,~pected due to a very low capacitance value of 3-5 pF measured for these lasers, which is one order of magnitude lower than for conventional BH lasers [1]. The dovetail geometry discussed above wi!l bring additional improvements in terms of threshold current and contact resistance.
6. Conclusion We have demonstrated MOVPE growth both at atmospheric and at reduced pressure of high quality InP and InGaAs layers and the doping characteristics for these materials. For the first time manganese doping is reported for MOVPE growth. We have developed the basic tools for monolithic integration of optoelectronic devices such as selective MOVPE resulting in planar surfaces and selective growth of semi-insulating InP layers with ultra high resistivities of p---2 × 10 9 Q cm. Discrete double-heterostructure bipolar transistors have been fabricated with the highest current gain of fl = 26000 reported to date for this material system. I n G a A s P / I n P lasers with semi-insulating InP current blocking layers and a planar surface have been realized. Threshold currents of 26 mA and extremely low capacitance values of 3-5 pF have been achieved.
Acknowledgements We wish to thank our colleagues C. Emele, A. Ifl~inder, B. Knapp, M. Klenk, W. Kuebart, G. Miiller, M. Schilling, I. SchiSttle, H. Schweizer, A. Stumpf, P. Wiedemann and O. Hildebrand for technical support and helpful discussions. Financal support by the European Strategic Programme for Research and Development in Information Technology (ESPRIT Project 263 B) and by the German Ministry of Research and Technology (BMFT) is gratefully acknowledged.
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P. Speier et a L / Monolithically integrated DH laser / HBT laser driver [7] G,B. Stringfellow, J. Crystal Growth 75 (1986) 91. [8] A.W. Nelson and L.D. Westbrook, J. Crystal Growth 68 (1984) 102. [9] C. Blaauw, B. Emmerstorfer and A.J. SpringThorpe, J. Crystal Growth 84 (1987) 431. [10] M. Schilfing, W. Kuebart, G. Miiller, G. Schemmel and F.J. Tegude, in: Proc. 16th European Solide State Device Research Conf., 1986, p. 129. [11] J. Long, V. Riggs and W. Johnston, J. Crystal Growth 69 (1984) 10. [12] P. Speier, G. Schemmel and W. Kuebart, Electron. Letters 22 (1986) 1216. [13] A. Macrander, J. Long, V. Riggs, A. Bloemeke and W. Johnston, Appl. Phys. Letters 45 (1984) 1297. [14] G. Iseler, in: Proc. 7th Intern. Syrup. on GaAs and Related Compounds, St. Louis, MO, 1978, Inst. Phys. Conf. Ser. 45, Ed. C.M. Wolfe (Inst. Phys., London-Bristol, 1979) p. 144.
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[15] A. Juhl and D. Bimberg, in: Pro¢. 5th Conf. on Semi-Insulating I I I - V Materials, 1986, p. 477. [16] S. Adachi and H. Kawaguchi, J. Electrochem. Soc. 128 (1981) 1342. [17] P. Speier, A. Nowitzki and G. Schemmel, presentation at the 2nd MOVPE Workshop, Cornell University, 1985. [18] C. Blaauw, A. Szaphlonczay, K. Fox and B. Emmerstorfer, J. Crystal Growth 77 (1986) 326. [19] P. Speier, K. Wiinstel and F.J. Tegude, Electron. Letters 23 (1987) 1363. [20] A, Clawson, C. Hanson and T. Vu, J. Crystal Growth 77 (1986) 334. [21] T. Sanada, K. Nakai, K. Wakao, M, Knmo and S. Yamakoshi, Appl. Phys. Letters 51 (1987) 1054. [22] F,J. Tegude et al., to be published. [23] J.R. Hayes, R. Bhat, H. Schumacher and M. Koza, Electron. Letters 23 (1987) 1298.