Elimination of mask undercutting in the fabrication of InP/InGaAsP BH devices

In the fabrication of buried heterostructure InP/InGaAsP lasers, mask undercutting during the mesa etching step is alleviated by a combination of steps which includes the epitaxial growth of a large bandgap InGaAsP cap layer (1.05 eV.ltorsim.E.sub.g .ltorsim.1.24 eV) and the plasma deposition of a SiO.sub.2 etch masking layer. Alternatively, the cap layer may be a bilayer: an InGaAs layer or narrow bandgap InGaAsP (E.sub.g .ltorsim.1.05 eV), which has low contact resistance, and a thin InP protective layer which reduces undercutting and which is removed after LPE regrowth is complete. In both cases, etching at a low temperature with agitation has been found advantageous.

BACKGROUND OF THE INVENTION 
This invention relates to semiconductor devices and, more particularly, to 
buried heterostructure lasers. 
Semiconductor diode lasers fabricated from the InGaAsP/InP materials system 
are currently of interest for application in optical communication systems 
operating at 1.0-1.6 .mu.m. Of the wide variety of possible laser 
structures which can be fabricated, those which utilize a real refractive 
index waveguide, such as a buried heterostructure (BH), have recently been 
shown to have advantages with respect to the absence of both 
self-pulsations in their outputs and kinks in their light-current 
behavior. [R. J. Nelson et al, Applied Physics Letters, 37, 769 (1980).] 
For most refractive-index-guided laser structures, the fabrication 
sequence usually requires an etching step to form a mesa which ultimately 
defines the lateral dimensions of the optical cavity and the active region 
of the laser. Since the transverse mode characteristics of this cavity 
depend on, among other factors, the cavity geometry and dimensions, it is 
important to exercise a high degree of control over these parameters 
during the etching process if high yields of single transverse mode 
devices are to be obtained. For example, typically the dimensions of the 
active region of a BH laser, which operates in the fundamental transverse 
mode, are 0.15 .mu.m in thickness and a maximum of 2.0 .mu.m in width. 
Associated with the problem of strict geometrical and dimensional control 
is the problem of mask undercutting. This undercutting is often 
unpredictable, leading to a loss of dimensional and geometrical control 
over the desired mesas. In the case of a BH laser, having a 2.0 .mu.m wide 
active layer, undercutting of only 1.0 .mu.m on each side of the mesa 
results in complete loss of the mesas on the wafer. Even in cases where 
the mask undercutting is predictable and can be allowed for, a large 
amount of mask overhang can lead to problems in later processing steps, 
such as LPE regrowth where local growth dynamics can be adversely 
affected. It is clear, therefore, that the etchant system and etching 
technique used to fabricate mesas for these laser structures must allow 
for precise geometrical and dimensional control with little or no 
undercutting. In addition, the etchant of choice should leave a 
contaminant-free surface which is smooth and free from pits or other 
defects which may lead to problems in subsequent processing steps. 
Bromine-methanol is one etchant which has been utilized successfully as a 
preferential etchant in the fabrication of InGaAsP/InP BH lasers [M. Hirao 
et al, Journal of Applied Physics, 51, 4539 (1980); R. J. Nelson et al, 
IEEE Journal of Quantum Electronics, 17, 202 (1981)] and 
buried-waveguide-heterostructure lasers [R. B. Wilson et al, International 
Electron Devices Meeting, Technical Digest, 370 (1980)]. In those reports, 
however, emphasis was placed on device results rather than on the mesa 
etching process and the mask undercutting problem. 
SUMMARY OF THE INVENTION 
We have found that wet chemical etching can be used successfully to 
fabricate InP/InGaAsP BH lasers without significant undercutting occurring 
during the mesa etching step. In accordance with one aspect of our 
invention, a large bandgap InGaAsP cap layer is epitaxially grown on top 
of the BH and a SiO.sub.2 masking layer is plasma deposited on the cap 
layer. The masking layer is patterned using conventional photolithographic 
techniques, and then mesas are etched preferably using low temperature 
Br-methanol with agitation. For limited undercutting and low contact 
resistance the bandgap E.sub.g of the cap layer should be 1.05 
eV.ltorsim.E.sub.g .ltorsim.1.24 eV. LPE regrowth of InP lateral 
confinement layers along the sides of the mesa completes the BH. 
Another aspect of our invention contemplates the use of a multilayer cap 
layer including an InGaAs layer, or a narrow bandgap InGaAsP layer, which 
provides for low contact resistance, and a thin InP protective layer, 
which reduces undercutting and which is removed after LPE regrowth is 
complete. In this case LPE regrowth includes first growing the usual InP 
confinement layers along the sides of the mesa and then growing a 
quaternary protective layer over the InP confinement layers so that the 
InP protective layer can be selectively etched away without attacking the 
confinement layers.

Common elements in different figures have been given identical reference 
numbers in order to facilitate comparison. 
DETAILED DESCRIPTION 
With reference now to FIG. 2, the buried heterostructure (BH) laser shown 
in fabricated by first epitaxially growing a double heterostructure (DH) 
wafer on a suitable single crystal substrate. In general, the epitaxial 
layers are then masked and etched to form elongated mesas, one of which is 
shown in end view in FIG. 1. The geometry of the mesa delineates the 
stripe shape of the active layer which is typically located near the neck 
of the mesa. Later, epitaxial regrowth of layers along the sides of the 
mesa surrounds the active layers with wider bandgap, lower refractive 
index material and completes each BH. Electrical contacts are applied to 
the top and bottom of the wafer, which is then diced, as by cleaving and 
sawing, into individual laser chips. Finally, each laser is mounted on a 
suitable heat sink (not shown). 
More specifically, consider the InP/InGaAsP BH laser of FIG. 2. This laser 
comprises an n-InP substrate 10 on which are epitaxially grown 
(illustratively by liquid phase epitaxy (LPE) the following, essentially 
lattice-matched layers in the order recited: an n-InP first cladding layer 
12, an unintentionally doped In.sub.1-y Ga.sub.y As.sub.x P.sub.1-x active 
layer 14, a p-InP second cladding layer 16, and a p.sup.+ -InGaAsP 
contact-facilitating cap layer 18. These layers form a double 
heterostructure (DH) wafer. The proportions x and y in the active layer 
are chosen according to the desired operating wavelength of the laser as 
described, for example, by Olsen et al, IEEE Journal of Quantum 
Electronics, QE-17, 131 (1981). 
In order to delineate elongated mesas of the type shown in FIG. 2 from this 
wafer, a SiO.sub.2 etch-masking layer is deposited on cap layer 18 and, 
using standard photolithographic techniques, is patterned to form a stripe 
mask 20 over each intended mesa. Etching with Br-methanol delineates the 
mesa and narrows the active layer 14 to less than about 2.0 .mu.m wide. 
(It is typically about 0.1-0.2 .mu.m thick). 
In accordance with our invention, the mesa is delineated without 
significantly undercutting the mask 20 by a combination of steps; namely, 
the cap layer 18 is made to have a bandgap E.sub.g in the range 1.05 
eV.ltorsim.E.sub.g .ltorsim.1.24 eV, and the SiO.sub.2 mask 20 is plasma 
deposited under particular conditions (described more fully later). Then, 
the mesa is etched using Br-methanol at a low temperature (preferably 
about 0.degree. C.) with agitation so as to increase the ratio of etch 
depth to undercut. Above the upper limit of E.sub.g, the contact 
resistance is undesirably high, whereas below the lower limit excessive 
undercutting occurs. 
Under these conditions, undercutting is no greater than about 0.5 .mu.m on 
a side, and the specific contact resistance is less than 10.sup.-5 
ohm-cm.sup.2. 
After etching the mesa structure shown in FIG. 1, InP layers 22 and 24 are 
regrown by LPE along both sides of the mesa so as to surround the active 
layer 14 with wider bandgap, lower refractive index material. In addition, 
the first layers regrown are p-InP layers 22 and the second layers are 
n-InP layers 24, thereby forming blocking p-n junctions 26 therebetween. 
That is, when the cladding layers 12 and 16 are forward-biased to cause 
laser emission from active layer 14, the blocking junctions 26 are 
reverse-biased so that current is constrained to flow primarily through 
the mesa and hence through the active layer. 
Forward-bias voltage and pumping current are applied to the device via a 
broad area metal contact 28 formed on the substrate 10 and a stripe 
geometry metal contact 30 formed on n-InP layers 24. The stripe is 
delineated by an opening in dielectric layer 32. The source of voltage and 
current is not shown. 
In an alternative embodiment of our invention, a BH laser of the type 
depicted in FIG. 5 is fabricated as follows. An InP/InGaAsP/InP DH (layers 
12, 14, and 16) is fabricated in essentially the same manner as described 
above. However, the cap layer 18' comprises narrow bandgap (E.sub.g 
.ltorsim.1.05 eV) p.sup.+ -InGaAsP or p.sup.+ -InGaAs rather than the 
higher bandgap (E.sub.g .ltorsim.1.05 eV) InGaAsP described above. In 
order to exploit the superior electrical contacting properties of the 
narrow bandgap material, a protective layer 19 of p-InP is grown over the 
cap layer 18'. Layer 19 reduces undercutting and protects layer 18' during 
subsequent mesa etching steps. In order to define the mesa, SiO.sub.2 
stripes 20 are plasma deposited onto the protective layer 19 as described 
above. Etching in Br-methanol results in the mesa configuration shown in 
FIG. 3. 
Next, LPE regrowth of InP layers along the sides of the mesa results in the 
formation of blocking junctions 26 as previously described and as shown in 
FIG. 4. In addition, however, protective layers 25 of InGaAsP are grown 
over the InP layers 24. The layers 25 enable the p-InP protective layer 19 
to be removed by a selective etchant such as 7M-12M HCl without also 
attacking the underlying InP blocking layers 22 and 24. The p-InP 
protective layer 19 is removed because it is difficult to make good 
electrical contact to this material, whereas a far superior electrical 
contact can be made to the underlying narrow bandgap layer 18'. The 
completed BH laser is shown in FIG. 5 where the broad area contact 28 and 
the stripe contact 30 have been applied in the manner described in 
reference to FIG. 3. 
EXAMPLE I 
The following example describes experiments that were performed to 
demonstrate the superior undercutting characteristics of the combination 
of a plasma deposited SiO.sub.2 mask and an LPE-grown InGaAsP cap layer 
having a bandgap E.sub.g in the range of approximately 1.05-1.24 eV. 
The substrate was Sn-doped (n.apprxeq.10.sup.18 cm.sup.-3) InP and had a 
surface orientation within about 1.degree. of the (001) or (111) plane. 
The dislocation density was determined from etch pitting studies to be 
about 8.times.10.sup.4 cm.sup.-2. The mesa-etching apparatus was a 100 ml 
beaker containing approximately 80 ml of 1% (by volume) Br-methanol 
solution and a perforated Teflon.TM. basket for holding the wafer (Teflon 
is a trademark of Dow Corning Corporation). Although both plasma deposited 
SiO.sub.2 and Si.sub.3 N.sub.4 etching masks were tried, we found that 
SiO.sub.2 masks deposited under the following conditions were preferred 
from an undercutting standpoint. 
A commercially available plasma deposition system (Plasma Therm PK-12) was 
used. The measured plasma RF power density was about 40-50 mW/cm.sup.2, 
the chamber pressure was about 500-1000 mTorr, and the substrate table 
temperature was about 200.degree.-300.degree. C. When gas concentrations 
of 3% silane in argon (324 sccm) and 100% nitrous oxide (420 sccm) were 
mixed in the chamber, the deposition rate was 670 .ANG./min. The resultant 
Si0.sub.2 films had a refractive index of 1.47.+-.0.015, an etch rate in 
BOE (6:1, NH.sub.4 F:HF) of 3200 .ANG./min., and low compressive stress of 
about 1.times.10.sup.9 dynes/cm. These SiO.sub.2 films were also found to 
produce less undercutting than SiO.sub.2 films deposited using other 
techniques such as sputtering. 
Using this plasma deposition procedure, 3000 .ANG. of SiO.sub.2 was 
deposited on the (001) surface of the wafer. Stripes and windows were then 
defined along each orientation ([110] and [1111]) using standard 
photolithographic techniques. These samples were then etched to a depth of 
4.0-5.0 .mu.m using a 1% (by volume) Br-methanol solution at an 
essentially constant temperature of about 0.degree. C. The resultant mesa 
and channel profiles were observed from the appropriate cleavage plane. 
Because crystallographic planes near the {111}A planes tend to etch the 
slowest, these planes tend to develop as etching proceeds. For this reason 
the planar crystallographic features found are thought to be closely 
relarted to {111}A planes, although the measured {001}/{111} interfacial 
angle of 61.degree..+-.2.degree. was somewhat larger than the expected 
value of 54.74.degree.. The difference may be due to effects of nearby 
vicinal planes, similar to results found for Br-methanol etching of GaAs. 
The mesa profile achieved using the etching techniques described here has 
been used to fabricate BH lasers of the type shown in FIG. 2. A mesa of 
the particular geometry shown in FIG. 1 allows for a narrow active layer 
stripe width (w) near the "neck" of the mesa, while maintaining a wider 
electrical contact region (a) near the top of the mesa. The geometric 
relation between the neck width (w), the neck depth (d), and the stripe 
width (a) is given by the expression: 
EQU w=a-2d/tan .theta. (1) 
where .theta. is the interfacial angle. Because the only depth parameter 
which is easily measured is the total etch depth (h), the relationship 
between the neck depth (d) and the total etch depth (h) must be determined 
in order to utilize Eq. (1) in a practical fashion. The experimental 
determination of this relation is described by the expression: 
EQU d=0.61h+0.33. (2) 
Eq. (2), when combined with Eq. (1), yields the relation betwen (a), (w), 
and (h), which is: 
EQU w=a-0.68h-0.37. (3) 
These expressions have been found to hold for multilayer InP/InGaAsP 
structures (such as a DH) as well as for InP substrate material provided 
that the same etching conditions (0.degree. C. with agitation) are used 
and essentially no mask undercutting occurs. Assuming an accuracy of 
.+-.0.15 .mu.m in the measurement of the parameters (h) and (a), the 
calculated accuracy in the determination of the parameters (d) and (w) 
from Eqs. (2) and (3) is .+-.0.10 .mu.m and .+-.0.18 .mu.m, respectively. 
In the development of Br-methanol etching for channel and mesa formation, 
we have paid considerable attention to the problem of mask undercutting. 
In the fabrication of BH lasers, mask undercutting can result in the loss 
of precise dimensional control over the mesa, leading to a poor yield of 
single transverse mode devices. In addition, mask undercutting is 
frequently nonuniform (along a BH stripe, for example), and large 
scattering effects may result from the widely varying lateral dimension of 
the active region. Finally, excessive mask undercutting may seriously 
influence the manner in which epitaxial layer growth takes place near a 
mesa or channel in a subsequent regrowth step and is, therefore, again 
undesirable. 
Two parameters which influence mask undercutting during the etching process 
are temperature and sample/solution agitation. SEM micrographs were taken 
of mesas etched to the same depth using a 1% Br-methanol solution at 
0.degree. C. (with agitation), and 25.degree. C. (with minimal agitation), 
respectively. The etching mask used for these samples was plasma deposited 
SiO.sub.2 as described above. Qualitatively, we found that in the case of 
the mesa etched at 25.degree. C. with minimal agitation the sidewalls tend 
to be significantly rounded, with a weak (111)A crystallographic feature 
and a total-etch-depth:undercut ratio of about 2:1. In contrast, the mesa 
etched at 0.degree. C. with agitation was characterized by a strong (111)A 
crystallographic feature and a total-etch-depth:undercut ratio in excess 
of 20:1. 
A third factor which has been found to influence undercutting is mask 
composition. As mentioned previously, we found that plasma deposited 
Si.sub.3 N.sub.4 etching masks tend to undercut more than plasma deposited 
SiO.sub.2 masks under identical etching conditions. This difference may be 
related to a stress-induced enhancement of the etch rate near the mask, 
since we have found a weak correlation between the degree of undercutting 
and the Si.sub.3 N.sub.4 deposition parameters. However, effects due to 
mask adhesion or surface-enhanced diffusion of the etchant cannot be ruled 
out. 
An additional factor which has been found to influence mask undercutting 
during the fabrication of BH lasers is the composition of the top p.sup.+ 
-InGaAsP cap layer normally grown on the DH for contact purposes. We find 
that a p.sup.+ -InGaAsP layer having a bandgap of 0.97 eV (lattice-matched 
to InP) tends to undercut significantly, whereas a p.sup.+ -InGaAsP having 
a bandgap of about 1.20 eV essentially eliminates this problem. More 
specifically, for undercutting of less than about 0.5 .mu.m on a side, the 
bandgap of the cap layer should be greater than about 1.05 eV, but for a 
specific contact resistance of less than about 10.sup.-5 ohm-cm the 
bandgap should be less than about 1.24 eV. 
EXAMPLE II 
The following example describes the use of the procedures of EXAMPLE I to 
fabricate a BH laser of the type shown in FIG. 2. 
On a (100)-oriented, Sn-doped, InP substrate (n.about.10.sup.18 cm.sup.-3) 
we used LPE to grow the following essentially lattice-matched, epitaxial 
layers in the order recited: a 6 .mu.m thick Sn-doped n-InP 
(n.about.2.times.10.sup.18 cm.sup.-3) cladding layer 12, a 0.2 .mu.m 
thick, unintentionally doped In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y 
(x=0.75, y=0.55, E.sub.g =0.99 eV) active layer 14, a 2.6 .mu.m thick 
Zn-doped InP (p.about.1.times.10.sup.18 cm.sup.-3) cladding layer 16, and 
a 0.7 .mu.m thick Zn-doped InGaAsP (p.about.4.times.10.sup.18 cm.sup.-3, 
E.sub.g =1.20) cap layer 18. 
To form mesas from this heterostructure, a 0.3 .mu.m thick Si0.sub.2 layer 
was plasma deposited on cap layer 18 and was patterned using standard 
photolithography to define stripe masks 20 which extended along the [110] 
direction. A solution of 1% Br in methanol was used to form the mesa of 
FIG. 1 with h=5.3 .mu.m and a=5.6 .mu.m, from which we used Eqs. (1)-(3) 
to calculate that w=1.63 .mu.m and d=3.56 .mu.m. Undercutting of mask 20 
was measured to be less than 0.2 .mu.m on a side. 
Next LPE was again used to regrow a 1.0 .mu.m thick, Zn-doped 
(p.about.8.times.10.sup.17 cm.sup.-3) InP blocking layer 22 and a 4.5 
.mu.m thick Sn-doped (n.about.2.times.10.sup.17 cm.sup.-3) InP blocking 
layer 24, both along the sides of the mesa so that the top of layer 24 was 
essentially co-planar with the top of the mesa. 
Standard evaporation was used to deposit a Au/Sn/Au broad area contact 28 
on substrate 10 and to deposit a Au/Zn stripe geometry contact 30 on the 
cap layer 18. Contact 30 was delineated by an opening in dielectric (e.g., 
SiC.sub.2) layer 32. 
After metallization was complete, the wafer was diced into laser chips 
using standard sawing and cleaving. In operation, these lasers exhibited a 
median room temperature c.w. threshold current of 28 mA. Oscillation 
occurred both in the fundamental transverse mode and in a single 
longitudinal mode at .lambda.=1.32 .mu.m. 
EXAMPLE III 
This example describes experiments which demonstrate the efficacy of using 
a narrow bandgap InGaAsP cap layer to facilitate contacting with an InP 
protective layer to limit undercutting. 
Once again (100)-oriented, Sn-doped (n.about.10.sup.18 cm.sup.-3) InP 
substrates were employed. A 0.7 .mu.m thick, Zn-doped 
(p.about.1.times.10.sup.19 cm.sup.-3) InGaAsP (.lambda.=1.55 .mu.m) layer 
was LPE-grown on the substrate, and a 0.25 .mu.m thick, Zn-doped 
(p.about.2.5.times.10.sup.18 cm.sup.-3) InP layer was grown on the InGaAsP 
layer. As described in EXAMPLE I, plasma deposited SiO.sub.2 mask stripes 
were formed along the [110] direction and the mesas were etched to a depth 
of 6.2 .mu.m using Br-methanol. The InP layer limited undercutting of the 
mask to less than 0.1 .mu.m on a side. 
Thus, this procedure followed by the regrowth and selective etching steps 
described with reference to FIGS. 3-5 can be used to fabricate a BH laser 
of the type shown in FIG. 5. 
It is to be understood that the above-described arrangements are merely 
illustrative of the many possible specification embodiments which can be 
devised to represent application of the principles of the invention. 
Numerous and varied other arrangements can be devised in accordance with 
these principles by those skilled in the art without departing from the 
spirit and scope of the invention. In particular, while the various 
aspects of our invention have been described in terms of light emitting 
diodes operating as lasers, it will be apparent that the invention is also 
applicable to light emitting diodes operating as sources of spontaneous 
emission (e.g., edge-emitting LEDs).