TuC.3 The Growth of MQW Planar Buried Heterostructure Lasers with Semi-insulating Blocking Layers by OMVPE

J.M.Jowett, D.J.Moule, A.P.Wright and A.T.R.Briggs Advanced Sources Department, STC Technology Limited,

Harlow, England. Introduction Buried heterostrudure (BH) lasers with semi-insulating (SI) current blocking layers are attractive devices to use for high speed operations. They have a reduced parasitic capacitance compared with lasers using p n junction blocking layers and therefore are capable of higher modulation rates. In order to be effective, current blocking layers in a laser structure must be highly resistive to double injection of electrons and holes from the n+ and p+ contacts. This paper describes the growth of suitable SI material for BH lasers, and its electrical characteristics under single and double camer injection conditions. The difficulty of regrowth of InP around mesas is discussed. It is necessary to grow a thick SI layer for maximum resistance, and to achieve a planar surface to enable electrical contact to be made to the device. Finally, the device characteristics of a laser with highly resistive planar SI layers are reported.

Growth of Semi-Insulating Indium Phosphide Semi-insulating indium phosphide can be produced by introducing a dopant with an energy level close to the middle of the band gap which also acts as a trap for injected camers. Iron and titanium are suitable dopants to use as they have been shown to produce energy levels in this region. Iron has a deep acceptor energy level in InP at Ec-0.66eV, which acts as an electron trap (12). Titanium is reported to act as a deep donor in InP, with an energy level just above the mid-bandgap at Ec-0.63eV and should therefore act as a hole trap (3).

Iron and titanium doping of InP has been examined using atmospheric pressure OMVPE; the apparatus has been described previously (4). Commercially produced ferrocene [Fe(C,H5)2] was used as the iron source, it was held in a bubbler at 0°C and used purified hydrogen as the carrier gas. Cyclopentadienyl cycloheptatrienyl titanium [CpTiC7H71 was used as the titanium source. It is believed to have a low vapour pressure, and was held in a bubbler at 50°C. Purified argon was used as the camer gas through the bubbler instead of hydrogen to prevent

at a temperature of 650°C and growth rate of O.O67p/minute. I the formation of an involatile hydride (5). Trimethylindium and phosphine were used to grow indium phosphide i

Electrical Characteristics I-V measurements have been performed on indium phosphide doped with titanium and/or iron, sandwiched between different combinations of p' and n+ contacts. Gold and titanium contacts were evaporated onto both sides of the wafers. Dots of 1 5 0 , 2 0 0 , 3 ~ diameter were fabricated by lift-off photolithography and then wet etched through the surrounding highly doped semiconductor contact layer to create electrical isolation between dots. Etching was terminated in the semi-insulating layer to prevent the formation of a leakage path between the contacts.

The I-V results are summarised in Table 1. It can be seen that InP highly resistive to an electron current can be produced if it is doped with either iron or titanium and zinc. Only InP doped with bdth iron and titanium proved to be highly resistive in the presence of a hole current. None of the individual layers were resistive to double injection of camers.

For the semi-insulating material to be used as blocking layers in a buried heterostructure laser, it must be resistive to double injection of carriers. Therefore a hybrid structure was designed. An iron doped InP layer was incorporated next to the electron injection source, the n+ contact layer, and InP doped with both titanium and iron was grown adjacent to the hole source, the p+ layers. The I-V characteristics of this structure were examined. Figure 1 shows that unfortunately the SI material produced was only resistive to single injection of holes or electrons, and extremely leaky under double injection conditions.

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Table 1: The Resistivities of SI Material with Different Contacts

InP Dopants

Contacts

n'InP p'GaInAs p%aInAs p'GaInAs p'InP

Bottom n'InP n+InP n'InP p'InP

TOP p+InP p'InP

n'InP

Iron Highly Resistive Highly leaky p - 10' R c m

Highly Resistive p - 10' ZhcQ

All resistivities measured at 1V.

Titanium

Iron 6 Titanium

SIMS analysis has been performed on both iron and titanium doped indium phosphide samples. Iron is easily incorporated into the layers at the electrical saturation limit of approximately lO"~m-~(2). Titanium is incorporated into the indium phosphide at a concentration of 1-2~10'~cm-~. However the titanium doped layers were found to be contaminated with silicon at a concentration of 5~1O'~cm-~ and iron at mid 10'6cm-3 resulting from impurities in the titanium source.

Leaky Leaky

Leaky

~ ~ 7 x 1 0 ' &km p=2x104 ~ h c m

Leaky Highly Resistive p - 10' a m

Undoped InP has a background electron concentration in the range 10'4-10'5cm-3. Iron acts as a deep acceptor and its concentration is made to greatly exceed the background donor concentration. Thus the iron traps the background electrons to produce SI material and can also capture further injected electrons to produce material resistive to an electron current. Injected holes are not significantly trapped. Similarly titanium, acting as a deep donor, should produce SI InP when it is codoped with zinc. The material should be resistive to a hole current if the shallow zinc acceptor concentration is greater than the electron background and it is doped with titanium at a higher concentration (6). Table 1 shows that the behaviour of the iron doped InP is as expected, but the behaviour of the InP co-doped with zinc and titanium is anomalous since this material traps electrons rather than holes. This anomaly might be explained if only a small fraction ( ~ 5 % ) of the incorporated titanium were electrically active, and thus the electrically active titanium concentration was similar to the zinc concentration. The results from the titanium doped samples are further complicated by the incorporation of contaminants from the source. The electrical activity of the contaminants and thus their effect on the titanium doped InP is indeterminate. As a result it has also not proved possible to determine the electrical activity of the titanium.

Zinc 6 Titanium

Iron 6 Titanium above Iron

Table 1 shows that the growth of material highly resistive to double injection of carriers was only achieved with the insertion of a layer of n+InP between the iron doped layer and the p+ contact layer. This effectively blocks the injection of holes into the iron doped region, so that it is only subjected to single injection of electrons, a condition under which it is highly resistive. A small increase in capacitance of the layers is the only penalty to this action. This structure was subsequently used as the blocking layers in all SI BH lasers which we have produced.

Highly Resistive Very Leaky

Highly Resistive Very Leaky Highly Resistive p - 10' Rcm p - 104 ~ ~ c m

p - 1 O ' a m

Growth of InP around Mesas In order to produce a BH laser capable of high speed and high power operation the blocking layers must be of low capacitance and high resistance. These properties may be maximised by increasing the thickness of the SI material and hence the height of the mesa to be overgrown. Growth around a tall mesa must also result in a planar surface if good contact is to be made to the device. Studies have been made of growth around mesas of different shapes, oxide widths and at atmospheric and low (1% torr) pressure to achieve planar secondary regrowth. Oxide stripes at 3 0 0 ~ intervals were used to define the top of the mesas. The oxide width was varied to enable different oxide undercuts to be achieved to determine the effect upon planarisation. Different etching techniques were also employed to change the shape of the mesa. The mesas were overgrown by InP with GaInAs marker layers to enable the evolution of the growing interface to be studied.

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It was found that both at atmospheric and low pressure voids were created undemeath the oxide if the overhang either side of the mesa was greater than - 1 p . Different shaped mesas were required for atmospheric and low pressure growth in order to achieve a planar surface. At atmospheric pressure, planar regrowth was achieved over a mesa with gently sloping sidewalls up to a height of 2 . 5 ~ . However these mesas were difficult to etch, especially to any greater height. At low pressure planar secondary growth was achieved over a mesa with vertical sidewalls and 3.4p.m tall, figure 2. The GaInAs marker layers in figure 2, show that if growth had been stopped earlier a planar surface would have been achieved. The same mesa was overgrown at atmospheric pressure. In this case voids were created at the sides of the mesa and growth occurred over the top of the oxide. Growth around this mesa is thus preferred at low pressure. The use of low pressure growth also confers other advantages. Firstly InP grows off the sides of the mesa, thus quickly covering up the active region, and reducing the number of defects introduced at this interface through long exposure to the gas ambient. Secondly the mesa is easier to etch and is more reproducible, hence this is the preferred technique for the growth of blocking layers for a BH laser.

Device results SI BH lasers have been produced using three epitaxial growth steps with iron doped InP blocking layers grown at both atmospheric and low pressure. The C.W. characteristics of a BH laser at 20°C with SI blocking layers grown at low pressure are shown in figure 3. Such a laser 6CQt.m long has given an external efficiency of 18% per facet, with a threshold current of only 7mA and an output power of over 2OmW. A device with SI blocking layers grown at atmospheric pressure and 4OOp.m long, has achieved a threshold current of 14mA with an external efficiency of 27% per facet.

Small signal measurements have been performed on SI BH lasers with blocking layers regrown at atmospheric pressure. Devices with bulk active layers at 2oOmA drive current have yielded-3dB bandwidths of 8GHz. MQW SI BH lasers have given-3dB bandwidths of 7.4GHz, at a drive current of 9 h A , as shown in figure 4. However we believe these results to be limited by the parasitic capacitance of the mount to which the lasers have been bonded, because as the drive current was increased the resonance peak moved to higher frequencies whilst the maximum position of roll off remained the same in all cases.

Conclusions Highly resistive iron doped semi-insulating indium phosphide has been achieved, >109Skm, and its suitability for use as blocking layers in a buried heterostructure laser have been assessed. Indium Phosphide is easily doped with titanium and SI material was produced, although the results are clouded by source contaminants. Planar regrowth of InP over tall mesas has been achieved at atmospheric pressure and more easily at low pressure. Buried heterostructure lasers have been produced with iron doped blocking layers. Threshold currents as low as 7mA and output powers over 2OmW have been achieved. Modulation bandwidths up to 8GHz have been measured.

Acknowledgements The authors would like to thank D.Ando of Queen Mary College, London for the preparation of the titanium source and A.Chew of Loughborough Consultants for the SIMS results. This work has been partly supported by the LINK Advanced Semiconductor Materials Programme.

References 1. 2.

3. 4. 5. A.G.Dentai, private communication. 6.

G.C.Valley, S.W.McCahon and M.B.Klein, J.Appl.Phys., 64, 6684 (1988). K.Nakai, O.Ueda, T.Odagawa, T.Takanohashi and S.Yamakoshi, Inst.Phys.Conf.Ser.No.91, Chapter 3, Heraklion (1987). C.D.Brandt, A.M.Henne1, T.Bryskiewicz, K.Y.Ko, L.M.Pawlosicz, J.Appl.Phys. 65,3459 (1989). A.T.R.Briggs and B.R.Butler, J.Crysta1 Growth, 85, 535 (1987).

D.G.Knight and W.Benyon, J.Crystal Growth, 102,249 (1990).

21 0

Oxide width = 2.6 pm Oxide overhano = 0.7 u m

v Total SI layer thickness = 5.2 pm

10-

Light output power (mw)

10 L- 0.01 I 0.1 1 10

Voltage (VJ

~~

100

Figure 1. Resistivity vs. voltage for Fe/Fe and Ti doped InP with n+ or p+ contacts Figure 2. Overgrowth of mesa with InP and GalnAs marker layers at low pressure

Blocking layen grown at atmospheric pressure

20° c/

/ Threshold current

External differential quantum efficiency

Device length

7 mA

18% per facet

9 1

600 Wm

-g; \ \ \ \ 1

0 2 4 6 8 10 Current (mA) Frequency (GHz)

Figure 3. CW L-l characteristics of SI MQW buried heterostructure laser with blocking layers grown at low pressure

Figure 4. Frequency response for MQW SI buried heterostructure laser

21 1