Surface emitting laser

Reduction of laser threshold in an electrically pumped vertical cavity laser is the consequence of interpositioning of an electrode layer intermediate the active, photon producing region, and at least one of the two Distributed Bragg Reflectors defining the laser cavity. The advance is a consequence of the lowered pump circuit resistance due to elimination of one or both DBRs--in particular, to elimination of the p-doped DBR--from the pump circuit.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention relates to a category of lasers including vertical cavity 
lasers, of design known as surface emitting lasers. Most important, such 
devices appear to satisfy the desire for integratable lasers--lasers to 
serve in Opto Electronic Integrated Circuits as well as in all-optic 
circuits. Contemplated integrated circuits may include 
electronics--generally semiconductor electronics--serving both for 
operation of the lasers and for other purposes. 
2. Description of the Prior Art 
Virtually from inception, the emergence of the laser raised expectations of 
widespread use in integrated circuits--both ancillary to electronic 
circuitry and in all-photonic circuitry. The development of the 
electrically pumped pn junction laser promised to satisfy the desire. 
Nevertheless, commercially expedient integrated lasers are not a reality. 
While there have been a variety of obstacles, I.sup.2 r loss coupled with 
high lasing threshold values are central. For specialized purposes, cooled 
circuitry might suffice; for general use a more economical approach is 
needed. 
Worldwide effort has addressed the very promising Surface Emitting Laser 
aka Vertical Cavity Laser, and the consensus is that this approach points 
the way to commercially feasible OEICs. It is likely prevalent SELs will 
be based on active regions containing one or more Quantum Wells although 
active regions based on bulk material are not to be discounted. References 
tracing introduction and recent development are: Y. Arakawa and A. Yariv, 
"IEEE J. Quantum Electron.", QE-21, 1666 (1985); Gourley et al, "Applied 
Physics Letters", 49 (9), 489 (1986) and J. L. Jewell et al, "Optical 
Engineering", 29, 210-214 (1990). 
Effort at this time is directed to an SEL structure consisting of a p-n 
junction active region in which photons are generated responsive to 
pumping current--an active region which in earliest work is based on 
"bulk", likely homogeneous composition, and which, in later work, makes 
use of Quantum Wells or of superlattice structure. The number of quantum 
wells, more generally the thickness of the active photon-producing 
material layer, inescapably dictates lasing threshold. Desired reduction 
in I.sup.2 r heating has led to a decreasing number of QWs, culminating in 
the 2- or the 1-quantum well structure of U.S. Pat. No. 4,999,842 dated 
Mar. 12, 1991. Most effective cavitation is due to the very high 
reflectivity resulting from use of Distributed Bragg Reflectors (with 
reflectivities well over 99%, e.g. for 20+pair mirror structures on both 
sides of the active region). See U.S. Pat. No. 4,999,842 describing a 
structure having a laser emission threshold at 7 microwatts/.mu.m.sup.2 
for DBRs of 24-pair, 1/4 wavelength (1/4.lambda.) layers of GaAs and AlAs, 
embracing an active region based on an 80 .ANG. active layer of In.sub.0.2 
Ga.sub.0.8 As (1/32 wavelength quantum well) emitting at 980 nm. 
While the described work has resulted in acceptable lasing threshold values 
in the active material itself, heating due to high series resistance in 
the SEL pump circuit--a circuit including a p-type DBR, the active region, 
and an n-type DBR--continues to be a problem. Total power efficiency and 
maximum power obtainable from SELs continues to be low compared to that 
obtainable from edge-emitting structures (about 5% efficiency and 1 mW 
power output for SELs vs. 30% and 100 mW) for edge emitting structures. 
Origin of the problem is largely the p-type DBR--of the high series 
resistance due to low hole mobility and the high optical absorption 
resulting from increased p-type doping introduced to reduce resistance. 
Extensive effort directed to this problem has resulted in optimization of 
layer-to-layer interfaces in the mirror (allowing Continuous Wave room 
temperature operation without heat sinks--but only at the indicated 
performance level). Other effort has taken the form of high p-carrier 
doping levels either throughout the DBR or at the lowermost level (Y. J. 
Yang et al, App. Phys. Letters, vol. 58, pp. 1780-1782 (April 1991 ) as 
well as reduced number of Bragg pairs (by partial, or even complete, 
substitution of Bragg layers by silver). Both approaches result in 
associated optical absorption to lower the differential quantum efficiency 
of the cavity. While the trade-off (of lower resistance for lower quantum 
efficiency) is a useful design consideration, the overall problem remains 
unsolved. 
SUMMARY OF THE INVENTION 
The invention is directed to electrically pumped p-n junction laser 
structures, exemplified by SELs in which I.sup.2 r heating is lessened for 
given laser output power by removing part or all of one DBR from the 
electrical pump circuit. Structures of the invention depend upon 
interpositioning of a layer electrode intermediate the p-type side of the 
active region and at least the major part of the DBR on that side of the 
structure. From the operational standpoint, preferred structures depend 
upon elimination of part or all of the other DBR--desirably of the 
entirety of both mirrors from the electrical circuit, and rely upon 
positioning of electrode layers on both sides of the active region--the 
latter, as described in detail, likely including passive layered material 
("primary spacers") of such thickness as to center active 
(photon-generating) material at the peak of the resulting standing wave 
(of the standing photonic wave during lasing). In accordance with known 
practice, reduction of thickness of the active material layer as so placed 
increases the efficiency with which generated photons contribute to the 
standing wave while decreasing loss due to absorption of the standing wave 
energy. Active regions, as made up of three layers--(1) spacer, (2) active 
material layer, (3) spacer--in an exemplary structure of 1/2.lambda. 
thickness. For most purposes, it is useful to regard such active region 
together with attendant embracing regions--including at least one 
electrode layer and, likely, a secondary spacer--as together defining a 
full .lambda. cavity. The preferred structure, including another electrode 
layer and, likely, another associated secondary spacer, is of the same 
total thickness--like of .lambda. thickness. Material included within such 
cavity may serve additional function--e.g. the secondary spacer may serve 
to enhance reflectivity, and, accordingly, may be considered part of an 
adjacent mirror. 
A prime operational advantage of SELs of the invention is due to design 
variations permitted by the basic teaching. In addition to structures in 
which the advantage is gained by elimination of all or part of the n-type 
DBR (as well as the prime offender--the p-type DBR), separation of 
electrical and optical function permits further performance advantage. 
DBRs, or DBR portions no longer included in the pump circuit, may now be 
optimized optically. One such modification entails elimination of 
photon-absorbing, significant impurity dopant from the excluded portion or 
the entirety of the DBR on the p-type side of the junction--of the 
(formerly) p-type DBR layers. This electrode layer is contacted laterally 
(relative to laser emission, which in accordance with usual terminology is 
described as vertical). In a preferred embodiment, a similar electrode 
underlies the active region, or, alternatively, a conventional substrate 
electrode is used, to pump the structure. Such excluded DBRs, perhaps of 
undoped semiconductor, perhaps of dielectric material are likely of 
resistivity of 10.sup.-1 ohm-cm or larger. 
A primary value of such electrode layers--at least of that replacing p-type 
DBR layers in the pump circuit--is lessened resistance in the pump 
circuit. Considerations discussed lead to any of a variety of electrode 
materials. From the performance standpoint, electrodes are desirably 
metallic, e.g. gold. This permits least electrode thickness (for 
prescribed conduction) so as to result in least photonic absorption (for 
invariant electrode placement as centered on a standing wave node). 
Fabrication is the primary consideration that may suggest non-metallic 
electrodes--crystalline lattice matching to allow epitaxial growth, e.g. 
of secondary spacer and DBR layers, may indicate use of semimetal or 
heavily doped semiconductor. While advantageous, e.g. in permitting 
epitaxial growth, such substitution reduces quantum efficiency due to the 
increase photonic absorption associated with increased electrode 
thickness. Greater layer-to-layer variation in refractive index permitted 
by non-epitaxial growth methods, e.g. by magnetron sputtering relieves a 
restriction on cavity efficiency, Q, to further improve operation. This is 
a factor in choice of electrode material--non-epitaxial DBR growth may 
even be advantageous with electrode of semimetal or semiconductor as well 
as of metal. At this time relatively small index variation in materials 
suitable for epitaxy lead to non-epitaxial DBR growth techniques for 
longer wavelength values--.lambda.&gt;1 .mu.m as measured in vacuum (e.g. at 
1.3 or 1.55 .mu.m). 
It is proper to consider the inventive teaching as permitting separation of 
optical from electrical considerations in the DBR design/fabrication. 
Accordingly, relaxation of the need for p- or even n-doping of the DBRs 
adds a degree of freedom. DBR fabrication, for example, by evaporation 
techniques, is uncomplicated by the need to introduce and control dopant. 
Now-permitted use of dielectric mirrors reduces absorption: firstly, due 
to reduced thickness permitted from higher layer-to-layer index variation 
and, secondly, due to minimization of carrier absorption. Such 
consideration may, in itself, dictate use of a second electrode layer 
under the active region to permit use of undoped DBRs, top and bottom. 
Improved structures of the invention will have a major effect on a variety 
of applications such as optical computing, optical interconnection, high 
speed laser printing, and in visible lasers as in displays. Resulting 
apparatus, likely based on OEICs containing laser elements described, 
constitute a significant part of the inventive advance. 
GENERAL COMMENTS 
It is convenient to describe the inventive teaching in context of SEL 
structures now receiving worldwide attention. The invention is somewhat 
broader, in being based on structures in which laser cavitation is of such 
direction as to have a significant component in the direction of 
electrical pumping. The observation, that absorption for properly placed 
electrode layers of thickness less than approximately 1/4 wavelength 
(perhaps as large as 0.3 wavelength) may be a minor concern, is of 
consequence for any such structure in which cavitating energy is, at least 
in part, transmitted through the electrode. Structures in which cavitation 
is designedly non-parallel to the electrical pumping direction may 
benefit. 
Devices are described as containing "pn junctions". In fact, a variety of 
considerations may lead to an active material layer which is nominally of 
intrinsic conductivity. The resulting junction may properly be referred to 
as "pin". Such variation is to be considered as included within general 
reference to "pn" or "junction", etc. 
The invention is not primarily concerned with the exact nature of mirrors 
based on distributed, cooperative in-phase, reflection due to index change 
between 1/4.lambda. mirror layers. While resulting structures indeed 
depend upon Bragg reflection, they may differ in detail from conventional 
DBRs. For example, it is not required that alternating layers be made up 
of periods of identical index pairs. Pairs may be of differing index to 
result in graded reflectance, and additional layers, e.g. to result in 
triplets or higher order periods, may be included.

DETAILED DESCRIPTION 
Nomenclature 
Description is expedited by definition of terminology used. This is of 
particular value in view of inconsistent use of many of the terms in the 
literature. 
Active Material Layer--that layer of the SEL primarily responsible for 
photo generation responsive to pump current. 
Active Region--layered portion of the SEL containing the active material 
layer, generally including sandwiching passage layers (primary spacer 
layers), as likely bounded by an electrode layer at least on one 
surface--likely the upper surface, this electrode thereby replacing the 
transitional p-type DBR in the electrical pump circuit. From the operating 
standpoint, where there is no lower electrode layer, the active region may 
usefully be considered as defined by the mirror--usually an n-type DBR on 
the underside. 
Primary Spacer--layer material within the active region embracing the 
active material layer. Usual design dictates that primary spacer layers 
are doped with significant impurity, p-type on one side of the active 
material layer, n-type on the other side of the active material layer, 
thereby defining or contributing to the pn junction required in 
contemplated electrically pumped laser structures of the invention. 
Layered material in addition to the active material layer rand spacer 
layers within the active region, e.g. serving as part of a DBR, is of 
conductivity type consistent with the junction--is generally p-type or 
n-type. For most purposes, additional layered material within the active 
region is considered as part of the primary spacer. 
Electrode Layer--that layer serving for biasing (for pumping) the active 
region at least on the p-type side of the SEL. Together with an optional 
paired electrode layer, or, alternatively, together with a conventional 
electrode layer on the underside of an n-doped DBR, they define the active 
region. 
Secondary Spacer--any passive material, serving little consequential 
function in terms of either photon generation or mirror function, outside 
of the active region--generally in contact with an electrode layer. As 
discussed, a secondary spacer is optional if the primary spacer is of low 
refractive index relative to relevant DBR layer--desirable if the primary 
spacer is of relatively high index so as to assure electrode layer 
positioning so as to correspond with an energy trough in the standing 
wave. 
Conception leading to the disclosure entailed insertion of the p-type 
electrode into a spacer, so resulting in "secondary" and "primary" 
spacers. First commercial devices may be of such design, although 
variation may be with a view to secondary considerations--for example, the 
secondary spacer may be of altered index to better complement reflectivity 
of subsequent DBR layers. 
Design Criteria 
Requirements of the SEL structure of the invention are discussed with 
reference to FIGS. 1 and 2. Essential features depicted are: active 
material layer 10, likely constituted of one or more quantum well layers, 
e.g. as described in U.S. Pat. No. 4,999,842 or, alternatively, of bulk or 
superlattice material. The improvement in power efficiency offered by the 
inventive approach increases the likelihood of commercialization of the 
Single Quantum Well structure of that patent, and it is useful to consider 
layer 10 as containing a single or small number of quantum wells. In any 
event, layer 10 is discussed as sandwiched between primary spacer layers 
11 and 12. These layers, while shown as constituted of continuous 
material--which may be of uniform or graded composition may include 
separately identifiable layers, e.g. layers 11 or 12 may include regions 
serving as DBR layers, or layer 10 may include regions serving as quantum 
wells in Multi Quantum Well structures. Compositional grading of layers 11 
or 12 may serve to advance secondary design criteria, e.g. reduction of 
electrical resistance between an electrode and the active material layer. 
Spacer layers 11 and 12 are doped with significant impurity--e.g. p-type 
and n-type, respectively. (While structures of the invention may depend 
upon an upper p-type region for forming the requisite junction, there is 
no longer a design disadvantage in reversing the junction, particularly 
for preferred structures in which both DBRs are excluded from the pump 
circuit.) It is convenient to consider the cavity as including: secondary 
spacer layer (or DBR layer) 16; electrode layer 14; primary spacer layer 
11; active material layer 10; primary spacer layer 12; and if present 
together with underlying electrode layer 15 and any secondary spacer layer 
13. The structure is completed by DBR mirrors 19 and 20. Arrows 21 and 22 
represent hole and electron flow resulting in operation. Conductive layers 
may be of metal, may be of semimetal, or may be semiconductive. Selection 
depends upon a variety of factors--generally selection based on tradeoff 
as between performance and ease/cost of fabrication. Energy conservation 
is favored by use of metal, e.g. gold, silver, titanium, or resistivity 
perhaps of the order of 10.sup.-6 -10.sup.-5 ohm-cm (in any event offering 
lower pump circuit I.sup.2 r than that of a circuit including the DBR 
mirror layers now removed from the electrical circuit). Experimental work 
has led to use of such a metal layer of thickness of perhaps 50-100 .ANG. 
(or of the order of hundredths of a 1/4 wavelength). Non-metallic 
electrode layers--semimetal or semiconductor (the latter containing up to 
10.sup.21 carriers/cc) are suitable particularly for shorter wavelength 
emission--e.g. 0.850 .mu.m. Layer thickness of .apprxeq.1/12.lambda. and 
1/4.lambda. for semimetal and semiconductor electrodes, respectively, 
yield conductance similar to that of metal electrodes. Since conductivity 
is higher for lower bandgap material, electrode thickness may be increased 
less than proportionally with longer wavelength for the same conductivity, 
thereby decreasing the absorption penalty and increasing advantageous use 
of semiconductor electrodes. 
Regions above electrode layer 14 and below spacer 12 (below electrode layer 
15, if present), most importantly consist of DBRs 19 and 20 (although 
secondary spacers 16 and 13 may lie within the DBRs. 
Other features shown in the figure represent preferred characteristics but 
may be omitted for reasons of economy. Such features include encircling 
thick conductive layer 17 as well as 18 to lessen impedance in the pump 
circuit. 
FIG. 1 is designed primarily to serve as a basis for the above discussion, 
and not to exhaustively represent physical configurations. Discussion is 
largely consistent with an active region 10 supporting a single 1/2 
wavelength standing wave and with a single quantum well placed 
accordingly--with such well at the center of region 10, thereby assuring 
placement at the crest of the standing wave. Variations which may serve 
economic and/or performance goals may entail bulk or superlattice material 
as well as MQW structures. The inventive advance, importantly, contributes 
toward increased lasing efficiency, and, so, increases likelihood of 
commercialization of single quantum well devices (of U.S. Pat. No. 
4,999,842). Nevertheless, design flexibility resulting from ability to 
define layers of tens of .ANG.--with wavelengths of thousands of 
.ANG.--permits construction of devices with active regions made up of 
successive quantum wells, or of successive bulk or superlattice material 
layers centered about successive crests. In general, a preference 
continues to exist for all active layer material being positioned at a 
single standing wave crest, so that MQW structures are likely to contain 
no more than the .apprxeq.6-7 QWs which may fit within .apprxeq.1/4 
wavelength about a peak. 
As elsewhere in this description, while quantum well structures are 
certainly preferred from the performance standpoint, other considerations 
largely pertaining to fabrication cost, may dictate use of bulk or 
superlattice active material. 
FIG. 2, a plot on coordinates of layer thickness, .theta., on the ordinate 
and intensity on the abscissa, depicts the standing wave for the center 
portion of a representative structure, 30. The structure shown consists of 
an active material layer 31 sandwiched between primary spacer layers 32 
and 33. Electrode layers 34 and 35, shown as broken lines, are in contact 
with the primary spacer layers. Layers 36 and 37 may constitute secondary 
spacers, or, alternatively, first DBR mirror layers. As discussed, use of 
secondary spacers serve, inter alia, to assure positional correspondence 
of electrode layers 34 and 35) with nulls in the standing wave 37--likely 
as required, e.g. on the p-type side of the structure when the primary 
spacer, e.g. layer 32 is of high refraction index, n, relative to the 
first DBR mirror layer. For instances in which the primary spacer is of 
low index relative to the nearest mirror layer, the secondary spacer may 
be dispensed with. Under such circumstances, layers 36 and/or 37 may be 
regarded as mirror layers. Regarding the mirror structures as prototypical 
DBRs, paired layers above secondary spacer 36 are of low, high, low, high 
index as corresponding with layers 39-42 in that order. If layer 36 serves 
as a DBR layer, it is of high index. The underside DBR is symmetrical so 
that layer 41 is of low index, etc. 
The active material layer 31 is of thickness corresponding with 
1/4.lambda., or: 
EQU .lambda..sub.0 /4n (eq. 1) 
in which: 
.lambda..sub.0 is the wavelength as measured in vacuum, 
n=refractive index of the active material. 
An electrode layer of gold or other highly conductive metal as used 
experimentally, affords useful conductivity in thickness of the order of 
100 .ANG. (perhaps 1/100th of a 1/4 wavelength as measured in the metal 
layer). Semimetal or heavily doped semiconductor electrode layer material, 
still affording sufficient conductivity for thickness below the 
.apprxeq.1/4.lambda., the permitted maximum, may serve in a thickness 
range of perhaps 1/10-1/12.lambda., or 1/2-1/4.lambda.. (As elsewhere in 
the description, such measurements are presented as illustrative and not 
as limiting--actual limits depend upon a number of factors, e.g., the 
lateral dimensions of the laser.) 
The absorption of a thin electrode layer (or of other film) centered on a 
null is: 
##EQU1## 
in which: .alpha.=absorption coefficient 
.gamma.=overlap factor in accordance with eq. 3 or eq. 4 
L=film thickness 
The overlap factor .gamma. of a film centered on a peak of the standing 
wave (centered on an antinode) is: 
##EQU2## 
in which: .theta.=layer thickness in radians from eq. 5. 
The overlap factor of a film centered on a null of the standing wave is: 
##EQU3## 
Absorption as calculated above is for the hypothetical condition of zero 
reflectivity for the electrode layer. In fact, a metallic electrode layer 
has significant associated reflectivity to result in increased pass 
length--likely as corresponding with .apprxeq.ten reflection 
incidents--for radiation which is but single pass for the hypothetical 
condition. Absorption loss increases linearly with pass length. 
Reflectivity for semimetal and semiconductor electrode layers, while 
finite, is less than that of contemplated metals. 
FIG. 3 depicts an SEL structure 50 containing an active region 51 and Bragg 
mirrors 52 and 53. DBR 52 is constituted of alternating low and high 
refractive index, 1/4.lambda., Bragg layers 54 and 55, and mirror 53 is 
constituted of similar low and high index layers 56 and 57 as discussed in 
the description of FIG. 2. FIG. 3A illustrates an inventive species in 
accordance with which the SEL provides for a thin, metallic electrode 
layer 58 separated from active region 59 by primary spacer 60. For the 
design shown, desired current flow is assured by relatively thick 
conductive layer 61 to which feed circuitry, not shown, is connected. 
Layer region 61 is in conductive contact with and encircles electrode 
layer 58 which latter is within the functional portion of laser 50. Layer 
62 atop electrode layer 58 is the secondary spacer layer, like primary 
spacer layer 60 of passive material--desirably included as discussed where 
primary spacer 60 is of low refractive index relative to that of the 
lowermost mirror layer of DBR 52. It is succeeded by Bragg layers 54 and 
55, and may, itself, also serve as a functioning layer of DBR 52. The 
arrangement shown in FIG. 3A depends upon lower electrode 63, which 
together with electrode layer 58, and by means of electrical circuitry not 
shown, electrically pumps the laser. The relative proportions shown are 
fairly representative of use of a semimetal, a heavily doped 
semiconductor, or a metal electrode layer 58. The design of FIG. 3A 
depends upon current blocking region 64 for channeling pump current 
through encircled active material layer 59. The structure is completed 
with alternating low and high 1/4 wavelength layers 56 and 57 on the 
underside, together constituting DBR 53,, and by substrate 65, the latter 
n-doped where serving as part of the pump circuit including electrode 63. 
The requisite pn junction within active material layer 59 is the 
consequence of p-doped spacer 60 and n-doped DBR 53 possibly similar to 
mirror 52, possibly dissimilar. 
FIG. 3B depicts a version of the inventive structure, similar to FIG. 3A 
dependent upon a second, relatively thin, likely metallic, electrode layer 
58' with its encircling thicker conductive layer 61', overlying (likely 
relatively thick) primary spacer 60'. The design permits elimination of 
underlying electrode 63 as well as use of an undoped lower DBR mirror 53 
possibly similar to mirror 52, possibly dissimilar. The structure of FIG. 
3B is otherwise as shown in FIG. 3A. 
FIG. 4 depicts an array of lasers 70 which may be of the detailed design, 
e.g. as shown in either of FIGS. 3A or 3B. Supporting substrate 71 may, in 
the instance of the species of FIG. 3A, have served for epitaxial growth 
of Bragg mirror 53. Other variations may entail removal of lattice-matched 
substrate following such epitaxial growth to permit replacement with 
mechanically preferable material e.g. for more dependable support. 
FIG. 5 is a schematic representation of a portion of an OEIC consisting of 
a laser 80, together with drive electronics 81 on common substrate 82. 
Biasing of structure 80 is by means of lead 83 contacting an upper 
electrode layer such as layer 58 of either of FIGS. 3A or 3B. A second 
current path not shown may contact a lower electrode layer, if present, 
or, alternatively, an underlying electrode such as electrode 58' of FIG. 
3. 
FIG. 6 consists of three plots. The first, on coordinates of current on the 
ordinate and distance on the abscissa, depicts standing wave 89 cycling 
from trough intensity of minimum current value (null) 90 to maximum 
current value (or crest) 91. Active material layer 92, in a properly 
designed structure, is centered at standing wave crest 91. The active 
region includes primary spacers 93 and 94 sandwiching active material 
layer 92 to, together, define a 1/2.lambda. thickness. Electrode layer 95, 
centered at null 90 completes the structure as represented. 
The second plot of FIG. 6, on coordinates of conductivity-compositional 
variation (in one instance variation of Al content in AlGaAs) on the 
ordinate and distance on the abscissa represents a structure of 
composition serving to produce the standing wave of the first plot of FIG. 
6. Based on GaAs, in this instance, of Al content ranging from 0% across 
the active material layer 92 to graded compositions of maximum aluminum 
content of 40% in the sandwiching primary spacers 93 and 94. Primary 
spacer 94 achieves a desirable cross-over between loop resistance and 
photonic absorption by means of increasing aluminum content within region 
96 (within the range of from 25-40 at. % for the example shown) and of 
decreasing aluminum content to a 0% or near-0% aluminum content within 
region 97, to decrease the conductivity barrier and increase current flow. 
Mirror layers 98 and 99, in this instance of .apprxeq.15% Al complete that 
part of the structure diagrammed. 
The third plot of FIG. 6 in ordinate units of refractive index, n, and in 
the same abscissa units of distance, show the variation in n attendant on 
the compositional changes shown in the second plot of FIG. 6. Numbering is 
the same as in the second plot of FIG. 6. 
The Table reports characteristics useful in comparison of modified designs 
(examples numbered 4-9) with the prior art designs (examples 1-3) as 
treated in the technical literature. It is clear that the inventive 
advance is usefully incorporated in a large variety of designs, some of 
which have not yet emerged. It is impractical to describe all such 
presently contemplated structures, much less to attempt to predict future 
designs. Full scope of the invention is defined by the appended claims. 
Design parameters heading the columns are those relevant to device design 
with regard to the thrust of the invention. Column headings are relevant 
primarily to examples 2 through 9 and are largely inapplicable to the 
reference, prior art device of example 1. Design parameters in the order 
of the columns are: 
"Material"--material relevant to the pump circuit (with reference to 
examples 1 through 5, designating doping level of semiconductor material 
in the p-side electrode layer of the circuit; with reference to examples 6 
through 9, designating semimetal or metal composition of the electrode 
layer). Example 9 provides for a second conducting layer on the n-type 
side of the SEL as well. Example 1, included for comparison purposes, 
pertains to a prior art "standard" structure including a 19 period 
vertically biased p-doped DBR within the pump circuit. 
"Thickness" is that of the upper electrode layer for examples 2-9. For 
example 1, it refers to the penetration depth of cavitating radiation into 
a GaAs-AlAs DBR. The third column lists values of resistivity of the upper 
electrode layer in units of micro-ohm-cm. 
Absorptivity, .alpha., is that of the electrode layer as expressed in units 
of cm.sup.-1. 
A is the single pass (or per-pass) absorption for cavitating laser 
radiation for the entire resulting SEL--including both mirrors. 
The next two columns report cavity quantum efficiency, .eta. also on this 
basis--first for mirror reflectivity of 99.0%; and in the next column, 
quantum efficiency on the same basis for mirror reflectivity of 99.8%. 
The next column reports values of electrical resistance, r, of the p-type 
side of the SEL structure--for examples 2-9 as excluding the upper 
DBR--i.e. of the p-type material and associated electrode layer. 
The final column reports the product of r and A.sub.p, the single pass 
absorption of the p-type side of the SEL. This produce is a useful figure 
of merit in evaluation of advantages realized by structures of the 
invention. 
Other relevant design information is well-known to those skilled in the art 
and its inclusion here is unnecessary. Such information is available from 
the literature, see, for example, references cited in "Description of the 
Prior Art". 
__________________________________________________________________________ 
Example 
Material 
Thickness 
.rho.(.mu..OMEGA.cm) 
.alpha.(cm - 1) 
A .eta.(.990) 
.eta.(.998) 
r(.OMEGA.) 
r .multidot. A.sub.p 
__________________________________________________________________________ 
1. p(5 .multidot. 10.sup.18) 
(0.5 
.mu.m) 50 .0030 
.77 .40 
2. p(5 .multidot. 10.sup.18) 
1 .mu.m 
10,000 
50 .0055 
.65 .27 8 0.04 
3. p(5 .multidot. 10.sup.18) 
3 .mu.m 
10,000 
50 .0155 
.39 .11 2.7 
0.04 
4. p.sup.++ (10.sup.20) 
550 
.ANG. 
1,000 
500 .0008 
.91 .67 16 0.005 
5. p.sup.++ (10.sup.21) 
300 
.ANG. 
300 1,500 .0007 
.93 .74 8 0.0016 
6. ErAs 200 
.ANG. 
50 10,000 
.0008 
.93 .71 2 0.0006 
7. W 50 .ANG. 
11 .0010 
.91 .67 3 0.0015 
8. Au 50 .ANG. 
5 .0006 
.94 .77 2 0.0002 
9. Au 50 .ANG. 
5 .0002 
.98 .91 2 0.0002 
__________________________________________________________________________ 
Examples 2 through 9 involve lessening of pump circuit due to elimination 
of the upper, or p-doped, DBR, while final example 9 is directed to a 
structure in which the lower DBR structure is eliminated as well. Examples 
1 through 8 make use of a "conventional" underside electrode--of an n-type 
contact electrically connected to the underside of the lower n-doped DBR 
as used in present SEL designs. Discussion is primarily in terms of laser 
emission at 0.85 .mu.m. Such discussion is, of course, illustrative 
only--likely significant commercial use is expected to soon take the form 
of emission at longer wavelengths, e.g. at 1.3 .mu.m and 1.55 .mu.m. As 
discussed, the inventive advance is particularly advantageous as practiced 
at such longer wavelengths. 
In all but example 9, the bottom n-type contact, e.g. electrode 63 of FIG. 
3, is as used in prior art SEL designs. Example 9, instead, utilizes a 
second electrode layer on the underside as well. Reported devices as 
operating at 0.98 .mu.m have utilized upper or p-type Bragg mirrors based 
on GaAs/AlAs typically conductivity doped to a level of 5.times.10.sup.18 
cm.sup.-3 and, accordingly, resulting in absorptivity, .alpha..apprxeq.50 
cm.sup.-2. Loss for such a standard p-type mirror is, accordingly, 
approximately 0.0025 (0.25%). This value is comparable to needed levels 
for acceptably low threshold current densities, and is attainable for SELs 
of 1-4 quantum well structure. Reduction in external differential quantum 
efficiency is expressed in terms of mirror reflectivity, R, and absorption 
loss, A, in accordance with the relationship: 
EQU .eta.=(1-R)/(A+1-R) (eq. 6) 
in which: .eta. is laser quantum efficiency (disregarding external pump 
circuitry) and other terms are as defined above. The simplified equation 
presented is valid for R values of .apprxeq.0.9 and better as qualify the 
inventive structures. 
Examples 2-9 of the Table set forth performance characteristics in which 
biasing on the top side of the SEL--on the p-type side of the SEL--depends 
upon transverse conductivity of an electrode layer, e.g. layer 58 of FIG. 
3A, with current introduced via the associated thick conductive layer 
61--to result in near-uniform current distribution over the p-doped 
spacer, e.g. layer 60. 
The table is described with reference to example numbers: Example 1--use if 
made of a "standard" p-type mirror as vertically pumped, incurring a loss 
of 0.0025, which as increased by the constant n-type per-pass absorption 
of 0.0005 (the experimentally determined value assumed constant for all of 
examples 1-8) results in the indicated total per-pass absorption, "A" of 
0.0030, and in cavity quantum efficiency, ".eta." of 0.77 and 0.40, 
respectively, for 99% and 99.8% mirror reflectances. These values of 
reflectance, experimentally at 0.98 .mu.m, corresponding with 13 and 17 
Bragg periods are appropriate for SELs of about four quantum wells and 
single quantum well, respectively. 
Examples 2-8 are based on use of an electrode layer on the p-type side of 
the laser while example 9 provides for such layer both on the p- and 
n-type side. For comparison purposes, all examples may be regarded as 
based on electrode layers as sandwiched between primary and secondary 
spacers. For some purposes, presence of the optional "secondary spacer", 
assuring correspondence of electrode layer position with a null on the 
standing wave, may be treated as the first DBR layer. Most effective 
operation is assured by use of such a layer of best optical properties and 
of refractive index to most effectively advance both objectives. 
The Table is, for most purposes, self-explanatory. It relates conductivity, 
and consequent permitted thickness of the electrode layer to properties 
determinative of SEL operation. It is seen by comparison of examples 2 
through 5, all relating to electrode layers of doped semiconductor, that 
decreasing resistivity, in permitting decreasing thickness, lessens 
per-pass absorption to, in turn, result in improved cavity quantum 
efficiency (ratio of emitted to cavitating radiation). Example 6 is based 
on an electrode layer of the semimetal, ErAs. Semimetal electrode layers 
as well as highly doped semiconductor electrode layers, offer a range of 
crystalline lattice parameters to permit epitaxially growth of subsequent 
layered material--of secondary spacer as well as DBR layers. Otherwise 
suitable electrode metals may have lattice parameters to permit epitaxial 
growth both of the electrode layer and of subsequent material. For 
example, NiAl as well as CoAl are adequately matched to GaAs. This 
consideration, primarily concerning fabrication, is of lesser consequence 
for longer wavelength devices (e.g. 1.3, 1.55 .mu.m) in which 
non-epitaxial growth techniques offer thinner, more effective DBR 
structures in any event. 
DETAILED DESIGN CONSIDERATIONS 
Extensive effort--both experimentation and study--have produced additional 
design considerations to significantly advance prospective commercial 
implementation. Results of the effort are reported in this section. 
Electrode Layer 
Optimal design of the electrode represents a cross-over as between the 
biasing/pump function and device operation. Primary considerations are: 
electrical, dictating minimal pump circuit resistance and, by itself, 
leading to thick electrode layers; and optical, particularly optical 
absorption, leading to thin electrode layers. From the material 
standpoint, three types of electrode layers are contemplated: 
Metal layers are generally desired from the performance standpoint. High 
conductivities permit thin layers, which are properly located at a null in 
the standing wave, interact with least photonic energy and permit low 
per-pass absorption (despite high levels of absorptivity). Gold has been 
found to be a good choice both for device performance and fabrication. 
Chemical stability permits considerable freedom in time in fabrication. 
Other considerations may suggest alternatives--higher melting points of 
tungsten and titanium may be an advantage--limits diffusion to permit 
higher fabrication temperature at some expense to performance. Silver, 
with its lesser absorption for shorter wavelength emission, leads to its 
consideration e.g. in the visible spectrum. 
Tabular examples 7-9 are based on 50 .ANG. layer thickness. While 
performance suffers somewhat, use of somewhat greater thickness, to 100 
.ANG. or higher, e.g. to .apprxeq.400 .ANG. may be desirable, e.g. in 
permitting less fastidious deposition and/or resulting in higher yield. 
Semimetal layers, in ongoing work, show improving device 
performance--approaching that of metal. Reduction in performance penalty 
for shorter wavelength emission may lead to its preference due to 
permitted epitaxial growth. Tabular example 6 is based on use of ErAs, 
which in layer thickness four times that of the metal layers of examples 
7-8, results in comparable operating characteristics. For such 
thicknesses, increased absorption distance for increased thickness for 
semimetal relative to metal is offset by the decrease in absorptivity 
resulting from reduced number of carriers. Other semimetals offer a range 
of lattice parameters to satisfy epitaxial growth for contemplated DBR 
materials. See T. Sands et al, "Stable and Epitaxial Metal/III-V 
Semiconductor Heterostructures", Materials Science Reports, vol. 5, no. 3 
(November 1990). Modification in composition of semimetal electrodes 
permits adjustment in lattice parameter with little or no effect on 
conductivity. As an example, scandium has been added to ErAs to to more 
closely match its lattice parameter to that of GaAs to result in Sc.sub.x 
Er.sub.1-x As--e.g. at X=0.32, matching is near-exact. From the electrical 
standpoint, available resistivities generally in the range of from 
3.multidot.10.sup.-5 to 10.sup.-4 ohm-cm suggest layer thicknesses of from 
100 .ANG. to 400 .ANG. for contemplated laser structures having 
symmetrical active regions of approximately 30.times.30 .mu.m, or less. 
Generally, carrier mobilities in semimetals are greater than those in 
metals, leading to performance-equivalent thinner electrode layers than 
earlier expected. 
Semiconductor materials may be doped sufficiently to serve as a third 
material variant on the electrode layer. Examples 4 and 5 are based on 
GaAs, p-doped to levels of the order of 10.sup.20 -10.sup.21 
carriers/cm.sup.3, e.g. as carbon-doped in accordance with the work of C. 
R. Abernathy--see, App. Phys. Let., vol. 55, no. 17, pp. 1750-2 (Oct. 23, 
1989). Such doping levels have resulted in per-pass absorption again 
comparable to that of devices using metallic electrode layers (compare 
example 5 with example 8). 
Substrate 
For most purposes, substrates are of minimal thickness still assuring high 
yield of epitaxially grown layers, although some minimal thickness perhaps 
250 .mu.m may be desired from a mechanical standpoint. This latter, of 
course, depends on device, IC, or wafer dimensions, presence or absence of 
other support, etc. Still other considerations give rise to use of a 
temporary substrate--e.g. to serve during fabrication/growth--to be 
removed/replaced with a view to device operation. Removal of GaAs after 
epitaxial DBR growth, followed by replacement with non-matching material, 
e.g. diamond or glass, is one example of many, in this instance providing 
increased transparency, over the .apprxeq.200-920 nm range, to permit 
emission through the underside, usually the n-type side, of the laser 
structure. For wavelength emission within the range of .apprxeq.920-1000 
nm, GaAs generally manifests adequate transparency. For still longer 
wavelength--.apprxeq.1000-1655 nm InP is usefully employed. 
Aside from growth requirements, device size and other physical requirements 
are prime determinants for substrate material/design for usual devices 
providing for top surface emission. 
Active Region 
Dimensional considerations, primarily of the included active material 
layer, has been considered (largely in description of FIG. 2). As there 
indicated, the primary function of photon generation with least absorption 
loss is optimized by an active material layer of a 1/4 wavelength in 
thickness as measured in the active material (.lambda./4n), sandwiched 
between primary spacer layers of relatively little absorption. Total 
thickness with sandwiching layers is again an integral number of 1/2 
wavelengths--in devices studied such active region is of one wavelength 
thickness. 
Composition choice of the active material is on bases well understood to 
the skilled artisan. A primary factor concerns emission wavelength. 
Categories of suitable materials extensively studied at this time are 
based on modified GaAs for longer wavelengths within the visible and 
near-visible spectra, and on modified InP for longer wavelengths extending 
further into the infrared spectrum. Exemplary active materials, suitably 
grown by epitaxy on substrates of such binary compositions, of 
intermediate ternaries, as well as on alternative compositions are known 
from a variety of references, for example, from above cited U.S. Pat. No. 
4,999,842. Such references report a number of active materials operated 
either as bulk or as quantum wells. For convenience, representative 
compositions are tabulated in terms of associated emission wavelength 
("QW" following the active composition identifies materials operated as 
quantum wells rather than bulk): 
______________________________________ 
Active Material .lambda.(nm) 
______________________________________ 
InGaAlP QW 630-650 
InGaAlP 660-670 
InGaP QW 655-700 
InGaP 670 
AlGaAs 700-800 
GaAs QW 800-860 
GaAs 880 
InGaAs QW 880-1100 
InGaAsP 950-1400 
InGaAs QW 1400-1600 
InGaAs 1655 
______________________________________ 
The active region, generally defined by bridging mirror structures 
(generally DBRs), or by electrode layer/s as present, also includes the 
primary spacers. Requirements placed on such spacer material are: from the 
performance standpoint, minimal absorption for cavitating laser radiation; 
from the fabrication standpoint, crystalline lattice parameters to permit 
epitaxial growth both of the spacer and of subsequent material (of the 
active material layer and/or secondary spacer or DBR layer). From the 
dimensional standpoint, thickness is such as to result in the required 
1/2.lambda. or integral number thereof (as allowing for any penetration, 
e.g. in adjacent DBR layers). 
Primary spacers, since within the electrical pump circuit, are doped p- or 
n-type as part of the functioning pn junction laser. Structures considered 
have sometimes been based on graded doping/conductivity with conductivity 
increasing to reach a maximum where in contact with an electrode layer. 
Such graded composition reduces electrical resistance at the 
spacer-electrode interface. 
Cavity Mirrors, likely DBRs, perhaps as including conventional reflectors 
to result in "hybrid" mirrors, (see U.S. Pat. No. 4,991,179, issued Feb. 
5, 1991) are well-known. Alternating layers of AlAs and Al.sub.x 
Ga.sub.1-x As, resulting in index change of .apprxeq.3.0-3.5, have been 
used for emission wavelengths below about 1 .mu.m (as measured in vacuum). 
As noted, epitaxial growth techniques are appropriate. For longer 
wavelength emission, e.g. at 1.3, 1.55 .mu.m, reported materials are of 
substantially lesser index change so that overall advantage makes use of 
non-epitaxial growth (e.g. of magnetron sputtering deposition) importantly 
of alternating layers of Si and SiO.sub.2 with index differences of 
.apprxeq.3.5-1.5--to permit the desired 99+% reflectivity for structures 
of perhaps 4 mirror periods. 
Reflectivity of a DBR interface is proportional to the quantity 
##EQU4## 
in which: n.sub.1 =refractive index of DBR layer of lesser index 
n.sub.2 =refractive index of DBR layer of greater index 
Study leads to devices which yield cavity reflectivity of &gt;99.9% for 28 
period DBRs of AlAs and 15% AlGaAs (Al.sub.0.15 Ga.sub.0.85 As) for 
emission at 0.85 .mu.m. Four period DBRs of Si and SiO.sub.2 yielded such 
reflectivity at 1.3 .mu.m. 
General Comment--Expected impact of the inventive teaching has resulted in 
extensive consideration/experimentation to result in a level of 
development of considerable sophistication. In a desire to meet the patent 
laws and to successfully survive subsequent court attack, every effort for 
full disclosure has been made. The very profundity of the teaching has 
complicated disclosure. Many aspects of the claimed invention at its 
present stage of development--in providing for contemplated problems, in 
optimizing operation terms of real conditions--represent complications to 
increase difficulty in description of the general advance. As an example, 
in-principle discussion of cavitation and related properties of e.g. 
.lambda. is complicated by secondary design considerations resulting from 
penetration of cavitating energy and multiple reflections within an 
electrode layer to result in an increase in optical length of the cavity. 
Experimentation and consideration have advanced to a level at which it is 
improper even to provide for cavity dimensions as so corrected. Designs 
which may gain in operation from electrode cross-sections of deliberately 
graded conductivity, deny even this non-simplifying assumption. This 
consideration is further complicated by difference in behavior based on 
electrode design as affecting interfacial reflectivity, absorptivity, and 
thickness. 
For the most part, terminology definition and discussion have been in terms 
of device design appropriate to and likely to be adopted for textbook 
description. In terms of the above example, the active region is initially 
described as uncomplicated by the electrode layer, while, in truth, 
correction must be made for its presence. The description has been in this 
general format--with initial discussion in terms of the hypothetical 
structure as initially disregarding such sophisticated, sometimes 
secondary, variations. Only in subsequent description has attention been 
paid to such matters. In similar manner, claim language is to some extent 
necessarily in terms of such hypothetical structure. The person skilled in 
the art will, upon reading this description, be fully understanding of the 
teaching and will be enabled to carry out the inventive teaching without 
undue experimentation as required by the patent laws.