The present invention is a hetero superlattice pn junction. In particular, the invention combines n and p type superlattices into a single pn junction having a bandgap sufficient to create high frequency (i.e. blue or higher) light emission. Individual superlattices are formed using a molecular beam epitaxy process. This process creates thin layers of well material separated by thin layers of barrier material. The well material is doped to create carrier concentrations and the barrier materials are chosen in combination with the thickness of the well materials to adjust the effective bandgap of the superlattice in order to create an effective wide bandgap material. The barrier material for the n and p type superlattices is different from the material used to form either of the two types of well layers. A particular embodiment of the present invention forms a first superlattice from n type doped ZnSe well layers and undoped ZnMnSe barrier layers and forms a second superlattice from p type doped ZnTe well layers and undoped ZnMnSe barrier layers. The first and second superlattices are merged into a hetero superlattice pn junction. The thickness and composition of the individual well and barrier layers can be modified to adjust the effective bandgap of the pn junction. Therefore, a wide bandgap diode is formed from previously incompatible materials.

FIELD OF THE INVENTION 
This invention relates generally to the field of semiconductor devices. In 
particular, this invention relates to light emitting semiconductor devices 
(LEDs). More particularly, this invention relates to an LED structure 
which emits a high frequency light. The frequency of emitted light is 
related to the effective bandgap of the semiconductor material which is 
increased by the structure of the device. 
BACKGROUND OF THE INVENTION 
Semiconductor materials are useful for making a wide variety of electrical 
and electro-optic devices. This is because of the band structure of the 
semiconductor material which makes up the device. Semiconductor materials 
have a conduction band and a valence band which make up the band structure 
of the material. The conduction band is a range of energy states for 
charge carriers (electrons) wherein a charge carrier existing in an energy 
state above a minimum value has the ability to quickly move around the 
material and conduct current. The valence band is a range of energy states 
for charge carriers (holes) wherein a carrier existing in an energy state 
below a maximum value has the ability to quickly move around the material 
and conduct charge. The minimum and maximum values are called the 
conduction band edge and the valence band edge respectively. The minimum 
conduction band edge is always greater than the maximum valence band edge 
in a semiconductor. The difference in energy between the conduction band 
edge and the valence band edge is called the bandgap. When carriers which 
have enough energy to be in the conduction band lose energy and make 
transitions between the conduction and the valence bands, the carriers 
give off light. The frequency of light emitted from the semiconductor 
device is proportional to the size of the bandgap. Therefore, the lost 
energy of the carriers can be converted into light having a specific 
frequency by tailoring the bandgap of the semiconductor material. 
In order to have carriers lose energy and make a transition from a 
conduction band to a valence band, carriers must first exist in the 
conduction band. One way in which semiconductor devices act as a switch in 
conducting current is to inject carriers from the conduction and valence 
bands of one type of semiconductor material into the conduction and 
valence bands of a second type of semiconductor material. That is, 
electrons are injected from the conduction band of a n-type material to 
the conduction band of a p-type material by applying a positive voltage to 
the p-type material with respect to the n-type material across the 
junction of the two types of material. Similarly, holes are injected from 
the valence band of the p-type material to the valence band of the n-type 
material by applying a positive voltage to the p-type material with 
respect to the n-type material across the junction of the two types of 
material. As a result, forward biasing a pn junction semiconductor device 
places electrons in the conduction band of the p-type material and holes 
in the valence band of the n-type material. These carriers are available 
to make the bandgap transition and emit light having a frequency 
proportional to the bandgap energy. 
The light emitting pn junction is useful because light is only emitted when 
the pn junction is forward biased, and therefore, an electrical signal in 
the form of a high or low voltage can be easily converted into an optical 
signal in the form of light or no light. An optical signal is useful 
because many materials have a distinct reaction to light and make optical 
storage devices which have a higher storage density than electrical 
storage devices. Further, optical communication is highly desirable 
because much more data can be transmitted over a single optical fiber than 
over an electrical connection. In either optical storage or optical 
communication, a means such as an LED for converting electrical signals to 
optical signals is a requirement for taking advantage of different optical 
devices. Prior art LEDs are made of a Group III-V compound such as 
GaAs.sub.1-x Al.sub.x wherein x is a mole fraction of aluminum and 
typically ranges between 0 and 0.5. LEDs are made of this compound because 
the GaAs.sub.1-x Al.sub.x system is relatively easy to dope and because 
the bandgap can be tailored to some degree. The GaAs.sub. 1-x Al.sub.x 
semiconductor material is easy to dope because it is relatively free of 
defects which trap the carriers and make the carriers from the dopant 
immobile. The bandgap can be tailored to some degree by increasing the 
amount of aluminum added to the III-V compound. As x is increased the 
bandgap increases to a maximum of approximately 2.2 eV. 
It is important to tailor the bandgap of the semiconductor material because 
as the bandgap gets greater, the energy of the light emitted gets greater 
and the wavelength of the light gets shorter. The shorter the wavelength, 
the more signals which can be transmitted and the more data which can be 
stored in an optical storage media. The problem with the GaAs.sub.1-x 
Al.sub.x semiconductor compound is that the maximum bandgap which can be 
developed in the material is approximately 2.2 eV (at room temperature). 
This bandgap corresponds to a yellow emitted light. It would be desirable 
to have a semiconductor material with a higher bandgap capable of emitting 
shorter wavelength light. The prior art has recognized this and attempted 
to use different compound materials which can be both appropriately doped 
and give the proper bandgap. One such attempt is to use group II-VI 
compounds for the wide bandgap. These compounds such as ZnSe have bandgaps 
of approximately 2.7 eV. 
The problem with such compounds is that they are not readily made into pn 
junctions. For example, ZnSe can be easily doped n-type but not p-type, 
and ZnTe can be easily doped p-type but not n-type. The problem is thought 
to be due to a self-compensation effect in which acceptor (donor) 
impurities are electrically compensated for by the creation of oppositely 
charged point defects. This results in the effective cancellation of the 
acceptor (donor) dopant. This effect is material dependent and especially 
pronounced in wide bandgap semiconductors. As a result, doping typically 
only works for one type of electrical conduction (i.e. p type doping for 
ZnTe or n type for ZnSe) even though some p type ZnSe devices have been 
demonstrated. Therefore, pn junctions in any one type of wide bandgap 
material are difficult to make. 
One way in which to avoid the self-compensation effect is to form a pn 
hetero-junction. In such a structure, the choice of n and p type materials 
is based on the ease of doping the materials and the compatibility of 
different lattice constants, among other considerations. The region where 
radiative recombination takes place is determined by the relative amount 
of injected carriers from one side to the other (i.e. electrons from the n 
to p side or holes from the p to n side). The relative injection of 
carriers is controlled by the size of the band offset and the carrier 
concentration in each side of the pn junction. It is also important to 
have close lattice constants between the two materials so that 
dislocations and defects in the lattice will not prevent proper injection 
of carriers across the pn junction. Hetrojunctions made of p-type ZnTe and 
n-type ZnSe satisfy the doping consideration. However, these materials 
have approximately a 7% lattice mismatch when combined in a pn junction. 
This large mismatch generates defects in the pn junction and reduces the 
carrier injection across the pn junction. Another problem with this 
structure is the band offset. The conduction band offset is smaller than 
the valence band offset. This allows substantially more electrons to be 
injected into the p-type material than holes injected into the n-type 
material. As a result, more electron recombination in the p-type region 
will be generated than hole recombination in the n-type region. This will 
produce a lower energy and longer wavelength emission because the p-type 
ZnTe has a lower bandgap than n-type ZnSe. 
Another prior art attempt to create a wide bandgap material employed a 
modulation doping technique to a short period, strained superlattice 
structure (SLS) in the ZnTe/ZnSe system. The technique demonstrated that p 
type conduction could be attained by doping the ZnTe layers p type with 
antimony (Sb) and interleaving the ZnTe layers with ZnSe layers. One 
period of the p type material has a 1 nm ZnTe layer and a 1 nm ZnSe layer 
and the p type layer has 300 such periods. The problem with this structure 
is that the hole concentrations is not high. Particularly, it is only 
approximately 10.sup.-3 /cm.sup.3, whereas practical device applications 
require hole concentrations of approximately 10.sup.17 /cm.sup.3. The low 
hole concentration means that the dominant radiative recombination will be 
electrons recombining in the lower bandgap p-type material. Moreover, band 
to band emission was not the dominant radiative recombination process 
because the intensity of photoluminescence peaked at approximately 2.006 
eV which is much lower than normal band to band recombination. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to manufacture light emitting 
semiconductor devices. 
It is another object of the present invention to manufacture semiconductor 
laser devices. 
It is a further object of the present invention to manufacture light 
emitting semiconductor devices which emit a high frequency light. 
It is still another object of the present invention to manufacture high 
frequency light emitting devices which efficiently generate light. 
It is still a further object of the present invention to manufacture high 
frequency light emitting devices which generate coherent light. 
It is still another object of the present invention to manufacture high 
frequency light emitting devices having improved manufacturability. 
SUMMARY OF THE INVENTION 
The present invention is a hetero superlattice pn junction. In particular, 
the invention combines n and p type superlattices into a single pn 
junction having a bandgap sufficient to create high frequency (i.e. blue 
or higher) light emission. Individual superlattices are formed using a 
molecular beam epitaxy process. This process creates thin layers of well 
material separated by thin layers of barrier material. The well material 
is doped to create carrier concentrations and the barrier materials are 
chosen in combination with the thickness of the well materials to adjust 
the effective bandgap of the superlattice in order to create an effective 
wide bandgap material. The barrier material for the n and p type 
superlattices is different from the material used to form either of the 
two types of well layers. A particular embodiment of the present invention 
forms a first superlattice from n type doped ZnSe well layers and undoped 
ZnMnSe barrier layers and forms a second superlattice from p type doped 
ZnTe well layers and undoped ZnMnSe barrier layers. The first and second 
superlattices are merged into a hetero superlattice pn junction. The 
thickness and composition of the individual well and barrier layers can be 
modified to adjust the effective bandgap of the pn junction. Therefore, a 
wide bandgap diode is formed from previously incompatible materials.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates the pn junction structure of a preferred embodiment of 
the present invention. FIG. 1 illustrates a multilayer structure in which 
layers 10, 30, and 40 are formed of a first semiconductor compound, layers 
60, 70, and 90 are formed of a second semiconductor compound, layer 80 is 
formed of a third semiconductor compound, and layer 20 is formed of a 
fourth semiconductor compound. Layer 50 may be formed of either the third 
or fourth type of semiconductor compound or it may be formed of a 
combination of the third and fourth types of semiconductor compounds. The 
first semiconductor compound is capable of being doped n-type and the 
second type of semiconductor compound is capable of being doped p-type. 
The third semiconductor layer is a barrier layer which is interposed 
between two p type layers. The p type layer 90 and the barrier layer 80 
form a first period 100 of the pn junction. The first period of the pn 
junction is replicated at least once and typically approximately 100 times 
to form the p type superlattice 200. The fourth type of semiconductor 
compound is a barrier layer which is interposed between two n type layers. 
The n type layer 10 and the barrier layer 20 form a second period 150 of 
the pn junction. The second period is replicated at least once and 
typically approximately 100 times to form an n type superlattice 250. 
Layer 50 separates the n type superlattice 250 from the p type 
superlattice 200. The n type superlattice 250 separated by the barrier 
layer 50 from the p type superlattice 200 forms the hetero superlattice pn 
junction of the present invention. In this particular embodiment, the 
barrier layers are intrinsic and not doped. The contacts 3 and 5 are 
formed on the n and p type superlattices respectively for applying a 
voltage across the pn junction. 
The first and second types of semiconductors are chosen for their ease of 
being doped either n or p type respectively. In particular, wide bandgap 
materials such as II-VI compounds, I-III-VI chalcopyrites, and II-III-VI 
chalcopyrites, are chosen for the characteristic of being capable of 
controlled doping of either n or p type. The third and fourth types of 
materials are chosen for their epitaxial compatibility, distribution of 
band discontinuities, lattice matching, and common element sharing with 
second and first types of semiconductors respectively. The third and 
fourth semiconductor materials are barrier layers interposed between the 
second and first semiconductor materials respectively. The barrier 
materials must have close enough lattice matching to avoid introducing too 
much strain into the superlattices. An acceptable amount of lattice 
mismatch is approximately 1-3%. Lattice mismatch above this amount causes 
unacceptable amount of carrier traps in the pn junction. The barrier 
layers introduce valence and conduction band discontinuities into the 
valence and conduction bands of the n and p type well materials of the 
superlattice. These band discontinuities must be tailored to achieve the 
required effective bandgap for efficient radiative recombination. For 
example, when the p-type bandgap is approximately 2.6 eV and electron 
recombination in the p-type region is the dominant mode of radiative 
recombination, the barrier layer must have a bandgap of approximately 3.0 
eV to create an effective bandgap of approximately 2.8 eV in the p-type 
material. The selection of a barrier layer material having a bandgap 
larger than the well material tailors the band edge discontinuities to 
increase the effective bandgap of the superlattice. 
FIG. 2 illustrates a band diagram for a hetero-superlattice according to 
the present invention. A first barrier semiconductor material A.sub.b 205 
is interposed between two layers of a first well material A.sub.w 207. The 
first well material 207 is doped n type and the barrier material is not 
doped. The conduction band is higher in energy in the barrier material 
than in the well material and the barrier valence band is lower in energy 
than the well valance band. The effective conduction band in the first 
superlattice 203 is higher than the conduction band in the well material 
but lower than the conduction band in the barrier material. Similarly, the 
effective valence band in the first superlattice 203 is lower in energy 
than the well material valence band but higher than the barrier material 
valance band. Also, a second barrier semiconductor material B.sub.b 215 is 
interposed between two layers of a second well material B.sub.w 217. The 
second well material 217 is doped p type and the barrier material is 
intrinsic. Again, the effective conduction band in the second superlattice 
210 is higher than the conduction band in the well material but lower than 
the conduction band in the barrier material. Further, the effective 
valence band in the second superlattice is lower than the valence band of 
the well material but higher than the valence band of the barrier 
material. The p and n type superlattices are joined into a pn junction 
hetero superlattice through barrier layer 50. FIG. 2 illustrates the 
effective energy band diagram at thermal equilibrium and no applied 
voltage. When a forward voltage is applied to the hetero superlattice, 
electrons are injected from the n type superlattice into the conduction 
band of the p type superlattice where they recombine radiatively through a 
sufficiently large bandgap energy to produce a high (blue or greater) 
frequency light. Holes are also injected from the p-type material into the 
valence band of the n-type material and recombine into the conduction band 
through a sufficiently large bandgap to produce a high (blue or higher) 
frequency light. 
The degree to which the effective conduction or valence band edge is 
different from the respective band edges in either the well or barrier 
materials alone, depends on the thicknesses of the well and the barrier 
materials. FIG. 3 illustrates the general relationship between the 
effective bandgap (difference between the valence and conduction band 
edges) for hypothetical superlattices A and B. Superlattice A has a well 
material having a relatively large bandgap energy and having a thickness 
equal to the thickness of the barrier layer. Superlattice B has a well 
material having a relatively small bandgap and having a thickness equal to 
the thickness of the barrier layer. As the thickness of the well and 
barrier layers for material A decreases, the effective bandgap rises. 
Material B has a lower bandgap for thick layers of well and barrier layers 
but this bandgap can be raised to that of material A when the thickness of 
material B is reduced a sufficient amount below the thickness of material 
A. FIG. 3 assumes that the thickness of the well material is the same as 
the thickness of the barrier material and this simple case is only 
explanatory. The well material may be thicker than the barrier layer or 
the barrier layer may be thicker than the well material depending on how 
the effective bandgap is to be adjusted. 
The effect of well and barrier layer thickness on the effective bandgap is 
important for two reasons. First, the effective bandgap of a superlattice 
can be adjusted by merely changing the thickness of the superlattice 
layers. Second, different superlattices can have their bandgaps adjusted 
by different amounts. This second effect is important because the relative 
alignment of the effective bandgap edges determines not only the 
wavelength of emitted light but it also determines the relative quantity 
of light emitted in the particular bandgap region. For example, if hole 
injection into the n-type material emits a low frequency light compared to 
electron injection into the p-type material, then the total emitted light 
from the device has low frequency components. The difference in frequency 
of emission between the n and p type superlattices can be reduced, as 
explained above, by making the well and barrier layers of the n-type 
superlattice thinner than the p-type superlattice. Also, this adjustment 
lowers the amount of holes injected into the n-type superlattice for a 
given forward bias which lowers the amount of low frequency components in 
the emitted light of the structure. 
The structure of FIG. 1 is built through the use of conventional epitaxial 
growth process equipment that has the capability to deposit thin layers of 
epitaxial material. Process equipment such as a conventional molecular 
beam epitaxial (MBE) growth system is preferred but other equipment such 
as a conventional organo-metallic vapor phase epitaxial growth system is 
sufficient. FIG. 1 illustrates that the pn junction hetero superlattice 
structure is formed on a four layer N+ buffer/substrate structure 300. The 
structure of FIG. 1 is built by loading an N+ doped GaAs &lt;100&gt; substrate 
302 into a molecular beam epitaxy chamber. The substrate 302 is heated to 
approximately 600.degree. C. to remove surface oxide from the substrate. 
The temperature is then reduced to 580.degree. C. and a GaAs buffer layer 
304 having approximately 300 nanometer (nm) thickness is grown over the 
GaAs substrate. Layer 304 is a GaAs layer doped with silicon. The silicon 
concentration is approximately 2.times.10.sup.18 /cm.sup.3. This is 
followed by the growth of an N+ Ga.sub.1-x In.sub.x As:Si buffer layer 306 
at 475.degree. C. The indium composition in this buffer layer has been 
monotonically increased from X=0 to X=0.23 and the thickness of this layer 
is approximately 700 nm. Again, the silicon concentration is approximately 
2.times.10.sup.18 /cm.sup.3. A third N+ buffer layer 308 is then deposited 
on the wafer which has a composition of Ga.sub.1-x In.sub.x As:Si wherein 
x is 0.23 and the thickness is 300 nm. The N+ dopant is added to all the 
buffer layers by opening the silicon effusion cell in the MBE chamber and 
maintaining the substrate 302 at 1050.degree. C. which yields an electron 
concentration of 2.times.10.sup.18 /cm.sup.3 in the buffer layer. After 
buffer layer 308 is grown, the temperature of the substrate 302 is reduced 
to approximately 300.degree. C. and the surface of the buffer layer 308 is 
exposed to Zn flux for approximately one minute. The concentration of the 
Zn flux is approximately 5.times.10.sup.14 /cm.sup.2 but the sticking 
efficiency of Zn is very poor so little of the Zn remains in the layer 
308. This process is performed to chemically modify the surface of the 
buffer layers to provide a smooth growing surface for the structural 
layers to follow. 
Once the buffer layers have been formed, the n type superlattice 250 can be 
grown. The superlattice 250 has 100 layers, 50 layers of ZnSe separated by 
50 layers of undoped ZnMnSe. The ZnSe layers are approximately 4 nm thick 
and range between 1 and 10 nm thick. The ZnMnSe layers are 1.5 nm thick 
and range in thickness from 1 to 5 nm thick. The alternating layers are 
formed at 300.degree. C. and composition control is maintained by opening 
and closing the effusion cell shutters of Mn and ZnCl.sub.2 while the Zn 
and Se shutters are kept open. The growth rate of ZnSe is 0.26 nm/sec. and 
the growth rate of ZnMnSe is 0.35 nm/sec. The composition of the ZnMnSe 
layer is approximately Zn.sub.0.7 Mn.sub.0.3 Se. The ZnCl.sub.2 cell 
temperature is set between 100.degree. and 120.degree. C. to give an 
electron concentration of approximately 1-5.times.10.sup.18 /cm.sup.3 in 
the superlattice 250 region. The growth of the p type superlattice region 
200 is started immediately after the last ZnMnSe layer has been formed. 
The superlattice 200 is formed of 100 alternating layers of phosphorus 
doped ZnTe and intrinsic ZnMnSe. Composition control of the superlattice 
200 is achieved by opening or closing the effusion cell shutters on the 
MBE system of Mn, Se, Te, and P while keeping the Zn shutter open. The 
temperature of the chamber remains at 300.degree. C. The phosphorus doped 
ZnTe layers are approximately 1 nm thick ranging between 0.5 and 5 nm in 
thickness. The ZnMnSe layers are also 1 nm thick ranging between 0.5 and 5 
nm in thickness. Also the ZnMnSe composition is the same in the 
superlattice 200 as it is in the superlattice 250. The resulting hole 
concentration in the superlattice 250 is 1-10.times.10.sup.17 /cm.sup.3. 
Once the superlattice 200 is formed over the superlattice 250 and over the 
buffer structure 300, the planar structure is ready to be formed into 
diode structures by conventional lithography and cleaving techniques. In 
particular, mesas are etched into the superlattice layers. The etch 
removes selective areas of superlattice structures to form a mesa of a pn 
junction hetero-superlattice structure on top of a buffer/substrate 
structure 300. Gold is evaporated onto the superlattice structure 200 to 
make a contact 5 to the p type superlattice 200. The gold electrode is 
approximately 50 nm thick and semi-transparent. The contact 3 to the n 
type superlattice 250 is made through the buffer/substrate structure 300 
by a metallic header. A positive voltage can then be applied to the gold 
electrode 5 with respect to the metallic header 3. The diode emits a blue 
light through the semi-transparent electrode at room temperature. Spectral 
analysis measures an emission peak at 480 nm. 
A second diode structure for the pn junction hetero-superlattice is shown 
in FIG. 4. In this case a Si.sub.3 N.sub.4 layer 7 is deposited over the 
superlattice 200. The layer 7 is then patterned and etched using 
conventional photolithography techniques. The pattern is an opening in 
layer 7 which is typically 800 microns long and 20 microns wide (although 
the width of the opening can vary greatly from several microns to several 
hundred or more microns). A layer of gold 5 is evaporated onto layer 7 and 
covering the etched opening. The gold layer is approximately 150 nm thick. 
The processed GaAs wafer is then cleaved into cavity structures having a 
length of typically 1.0 millimeter and a width of approximately 0.6 
millimeters. These cavity structures are then mounted on a copper heat 
sink which forms the contact 3 to the N+ type buffer/substrate structure. 
Again, positive voltage is applied to the gold contact with respect to the 
N+ contact 3 to forward bias the pn diode. Under forward bias condition 
with a threshold current density of approximately 625 A/cm.sup.2 the line 
width of the emission spectrum becomes very narrow, and coherent blue 
light emission occurs from the cleaved sidewalls. 
Alternate embodiments of the present invention can be fabricated by forming 
the first and second superlattice structures with alternative materials. 
In particular, the superlattice bandgaps are tailored by changing the 
thickness of the well and barrier layers and by changing the materials 
from which the well and barrier layers are formed. FIG. 3 illustrated the 
general relationship between the well and barrier layer thicknesses and 
the bandgap. This general relationship can be used when selecting from a 
variety of materials for making the well and barrier layers. A specific 
example of selecting compatible materials for the well and barrier layers 
is given below: 
A first 0.5 micron buffer layer of n-type ZnS.sub.-x Se.sub.x doped with Cl 
grown on an N+ Si&lt;100&gt; substrate, wherein X is monotonically graded from 0 
to 0.5. A second 1 micron buffer layer of ZnS.sub.0.5 Se.sub.0.5 doped 
with Cl grown on the first buffer layer wherein the electron concentration 
of these buffer layers is 2.times.10.sup.18 /cm.sup.3. A first 
superlattice comprised of 30 ZnSe well layers having a thickness of 
approximately 1.5 nm and 30 barrier layers of ZnS having a thickness of 
approximately 1.5 nm. The well layers are doped with Cl simultaneously 
with the growth of the layers. The first superlattice is grown on top of 
the barrier layers. A second superlattice is grown on top of the first 
superlattice. Immediately following the formation of the first 
superlattice which is terminated by an ZnSe well layer, a barrier layer of 
ZnS.sub.0.75 Te.sub.0.25 having a 1 nm thickness starts the formation of 
the second superlattice. The second superlattice has 30 well layers of 
ZnSe.sub.0.7 Te.sub.0.3 and 30 barrier layers of ZnS.sub.0.75 Te.sub.0.25. 
The p-type well layers are doped with nitrogen or phosphorous to a 
concentration of approximately 1.times.10.sup.17 /cm.sup.3. The Si wafer 
then has an approximately 50 nm thick semi-transparent gold layer 
evaporated on it to form one electrode. The Si wafer is then 
conventionally etched to form mesa diode structures having approximately a 
one millimeter diameter. Application of approximately 10 volts of forward 
bias between the N+ silicon and the gold contact results in emission of 
light having an emission peak of approximately 420 nm. 
Other examples of heterjunction systems are formed by combining n-type well 
layers in a first superlattice with p-type well layers in a second 
superlattice wherein each superlattice has compatible barrier layers. 
Suitable n-type well materials are: ZnSe, ZnS, and CdS (each doped with 
Al, In, Ga, Cl, Br, I, or F) or their alloys such as Zn(Se,S), (Zn,Cd)S, 
(Zn,Cd)Se, (Zn,Cd)(Se,S). Suitable p-type layers are ZnTe (doped by As, 
Sb, P, N, Bi, Cu, Ag, Au, Li, or Na); ZnTe based alloys, such as Zn(SeTe), 
(Zn,Cd)Te, Zn(Te,S), (Zn,Cd)(Te,S), and (Zn,Cd)(Te,Se); ZnSe (doped with N 
or Li); and ZnSe based alloys such as (Zn,Cd)Se and Zn(S,Se). Suitable 
barrier layers are: (Zn,Mn)Se for n or p type well layers in a 
superlattice; (Zn,Mn)(Se,Te) for p type well layers in a superlattice; 
(Zn,Mn)(S,Se) for n type well layers in a superlattice; and (Zn,Mn)(S,Te) 
for p type well layers in a superlattice. Barrier layers which do not 
contain Mn are Zn(S,Se) for n type well and Zn(S,Te) for p type well 
layers. In the case of no Mn, the S substitutes for group VI sites instead 
of Mn substituting for group II sites which boosts the superlattice band 
gaps of both n and p type materials. 
A further embodiment of the present invention uses I-III-VI type 
Chalcopyrite in combination with a II-VI group compound. Specifically, a p 
type superlattice has CuAlS.sub.2 as a well layer and undoped ZnS as a 
barrier layer. The n type superlattice has doped ZnSe as a well layer and 
ZnS as a barrier layer. More particularly, an MBE tool is used to grow an 
n type ZnS.sub.0.5 Se.sub.0.5 as a buffer/substrate layer on an n type 
silicon substrate. Alternating layers of ZnSe and ZnS are then deposited 
which form approximately 100 periods of ZnSe/ZnS. The ZnSe layers form the 
well layers and are doped with Cl to a concentration of approximately 
1.times.10.sup.19 /cm.sup.3 and are approximately 1.5 nm thick. The ZnS 
barrier layers are undoped and are approximately 1.5 nm thick. The n-type 
superlattice formed by the 100 periods of ZnSe/ZnS is ended by a ZnS 
barrier layer. A p-type superlattice is then formed over the n-type 
superlattice. The p-type superlattice has p-type well layers formed of 
CuAlS.sub.2. The well layer has a bandgap of approximately 3.4 eV, is 
doped p-type by Zn to a concentration of approximately 1.times.10.sup.19 
/cm.sup.3, and has a thickness of approximately 1.5 nm. The barrier layer 
is undoped ZnS and has a thickness of approximately 1.5 nm. The p-type 
superlattice consists of approximately 100 periods of well and barrier 
layers. Contacts are formed on the n-type buffer/substrate and p-type 
superlattice structures in a similar manner as shown in FIG. 1. 
Application of a forward bias is expected to exhibit radiation in the 3.0 
to 3.5 eV energy range. 
A further embodiment of the present invention dopes the barrier layers 
rather than the well layers as illustrated above. In particular, the 
effective bandgap of a doped material in a superlattice is raised by 
introducing a larger bandgap barrier layer which is compatible with the 
well material into the superlattice. Similarly, when the barrier material 
of a superlattice is doped, an undoped well material having a lower 
bandgap will decrease the effective bandgap of the superlattice. The well 
material introduces discontinuities in the wide bandgap structure which 
lower the effective bandgap of the superlattice. The overall effect of the 
hetero-superlattice structure remains the same. When the well and barrier 
layers are compatible in lattice structure (having a lattice mismatch of 
less than approximately 1-3%), then the source of the carrier 
concentration may be either from the well or the barrier for n or p type 
superlattices. 
While the invention has been described and illustrated with respect to 
plural embodiments thereof, it will be understood by those skilled in the 
art that various changes in the detail may be made therein without 
departing from the spirit, scope, and teaching of the invention. 
Therefore, the invention disclosed herein is to be limited only as 
specified in the following claims.