EP1228557A2

EP1228557A2 - Long wavelength pseudomorphic inganpassb type-i and type-ii active layers for the gas material system

Info

Publication number: EP1228557A2
Application number: EP00991716A
Authority: EP
Inventors: Shane Johnson; Philip School of Electrical Engineering DOWD; W. Paul-Drude Inst. for Solid State Elect. BRAUN; Yong-Hang Zhang; Chang-Zhi Guo
Original assignee: Arizona Board of Regents of University of Arizona; University of Arizona
Current assignee: Arizona Board of Regents of University of Arizona; University of Arizona
Priority date: 1999-11-01
Filing date: 2000-11-01
Publication date: 2002-08-07
Also published as: WO2001033677A3; WO2001033677A2; WO2001033677A9; KR20020059663A; CN1384990A; JP2003513476A

Abstract

The invention discloses improved structures of light-processing (e.g., light-emitting and light-absorbing/sensing) devices, in particular Vertical Cavity Surface Emitting Lasers (VCSELs), such as may find use in telecommunications applications. The disclosed VSCAL devices and production methods provide for an active region having a quantum well structure grown on GaAs-containing substrates, thus providing processing compatibility for light having wavelength in the range 1.0 to 1.6 νm. The active region structure combines strain-compensating barriers with different band alignments in the quantum wells to achieve a long emission wavelength while at the same time decreasing the strain in the structure. The improved functioning of the devices disclosed results from building them with multicomponent alloy layers having a large number of constituents. The invention discloses as a key constituent in the proposed alloy layers for the active region a substance, such as nitrogen (N), suitable for reducing bandgap energy (i.e., increasing light wavelength) associated with the layers, while at the same time lowering the lattice constant associated with the structure and hence lowering strain.

Description

LONG WAVELENGTH PSEUDOMORPHIC InGaNPAsSb TYPE-I AND TYPE-H ACTIVE LAYERS FOR THE GAAS MATERIAL SYSTEM

Vertical-cavity surface-emitting lasers (VCSELs) operating at 1.3 μm and 1.5 μm

are desirable for low cost optical telecommunication systems and data links. Realization of these

devices may enable digital communications applications such as "fiber to the home," which

operate over distances of only a few kilometers. Whereas lasers operating at a wavelength 1.3

μm are interesting for high-speed communication since they operate at the dispersion minimum

of optical fibers, lasers at 1.55 μm are interesting for communications over larger distances as

they transmit in the absorption minimum. Also, long-wavelength lasers have a low operating

voltage, which makes them attractive for integration with integrated Si-based circuitry in which

the trend has been toward lower and lower operating voltages with higher integration density.

Due to the potentially-large market for 1.3 and 1.5 μm VCSELs, much research has been carried

out to develop devices using different approaches based primarily on two substrates, InP and

GaAs. Whereas InP has been the traditional substrate material for edge-emitting lasers, GaAs

offers advantages in terms of lower substrate cost and potentially higher device performance.

In general, VCSELs are light emitting semiconductor devices including two

Distributed Bragg Reflectors (DBRs) between which lies an active region possessing a material

emitting the desired wavelength of light. A typical VCSEL structure is shown schematically in

Figure 1. In this case, the active region consists of several InGaAs quantum wells separated by

GaAs barriers and illustrates the general conduction band-edge alignment required in an active

region. The semiconductor structure is designed to have the minimum separation between the

electrons and the holes in the active zone, where the two carrier types recombine and emit light.

The wavelength of the emitted light is determined by the energy separation between electrons

and holes in this active zone. The particular active region shown is designed for emission at

about 980 nm, but the same design procedures are required at longer wavelengths also. An Al-Ga^As spacer is used to define the cavity length, which matches a multiple of half the

wavelength of the lasing wavelength λ. The composition is shown along with the relative

refractive indices and bandgaps of the different layers. Since the active region in a VCSEL is

short, typically significantly less than the lasing wavelength (λ), photons passing through the

active region experience only a small single-pass optical gain. Therefore, to achieve lasing

action, highly reflective structures are required on both sides of the active region. This may

readily be achieved by using the same epitaxial growth process as required to fabricate the active

region, or by techniques such as dielectric deposition. The mirrors consist of alternate λ/4 layers

of materials with different refractive indices. At the lasing wavelength, the partial waves

reflected at the interfaces between these layers interfere constructively, resulting in a very high

reflectance within a narrow spectral region. The thin-film stacks on both sides of the active

region form so-called distributed Bragg reflectors (DBRs), with typical reflectivities of 99% or

higher.

A waveguiding structure for the optical mode is required, for instance in the

configuration illustrated in in Figure 2. In an index-guided device as shown in Figure 2, the

optical mode is confined by etching away the material around a pillar-shaped volume to form an

air-post device. The current is then also confined to the pillar area. Alternatively gain guiding

may be used. Very high resistivity regions may be created by firing high energy protons or ions

into the device. This defines an area through which the current funnels into the active region.

The gain region is confined laterally, and the mode forms in the free area. A combination of

these schemes may also be used. A more recent development is the oxide-confined device. Selectively oxidizing mirror layers to form Al O,. cladding layers provides both current

confinement and index guiding, which can result in devices with very low threshold currents.

The operating principle of a light emitting structure can be reversed. If light of

the appropriate wavelength is directed on such a device, a current is generated at the two

terminals, allowing the operation as a detector. For high performance detectors, different

optimizations are needed, making the optimal structure different from an emitter. A typical

structure is shown in Figure 3. The active regions for both the light emitter and the detector,

however, use the same general type of layer structure.

VCSELs require laser-quality active materials and high reflectivity DBR mirrors.

Problems commonly encountered in the production of prior art VCSELs have been extensively

reviewed by U.S. Patent Nos. 5,719,894 and 5,719,895, which patents are hereby incorporated in

full herein. Generally, the production of VCSELs emitting in the region of 1.3 to 1.55 μm may

be complicated by the following problems:

(1) production of efficient DBRs for InP substrates is difficult and in

practice has been found to be very ineffective;

(2) VCSELs grown using InP/InGaAsP have poor performance due to

the materials' high thermal sensitivity and refractive index properties; and

(3) growth of laser-quality active material on GaAs has generally

proven to be unsuccessful for prior art VCSELs.

The materials used for lattice-matched mirrors on InP substrates are InP and

InGaAsP. These materials have shortcomings as well, viz.:

(i) a small refractive index step; and

(ii) poor thermal properties. The small refractive index step manifests itself in the number of layers required to

produce a DBR mirror with the desired reflectivity. When compared to AlGaAs stacks grown on

GaAs, stacks grown on InP require more InP/InGaAsP layers to produce the same reflectivity.

Additionally, InGaAsP displays a higher thermal resistance than GaAs or AlAs. This increases

thermal problems of the device — e.g., heating of the active zone--, making it more difficult to

achieve reliable continuous wave (CW) operation at room temperature. This problem is further

exacerbated because more material is required in the mirrors, thus increasing the distance over

which thermal conduction has to take place and at the same time increasing the heat-generating

volume if the operating current is passed through these regions.

Whereas GaAs may offer significant advantages in terms of lower substrate cost,

simpler crystal growth technology and higher reflectivity mirrors, the problem of growing high

optical quality active material on GaAs is a problem that many researchers have attempted to

tackle in numerous ways. One common method of solving this problem is through the use of

gain offset as disclosed by Jewell et al. in U.S. Patent Nos. 5,719,894 and 5,719,895, which are

incorporated herein by reference. Although the methods outlined by Jewell et al. appear to have

some attractive features, it is still generally found to be extremely difficult to achieve the desired

wavelength emission using such methods because process parameters such as critical thickness

are not sufficiently developed. Specifically with regard to U.S. Patent No. 5,719,894, the

following problems tend to be encountered:

1. As taught by Jewell et al., the incorporation of nitrogen (N) into

semiconductor material, such as InGaAs to form InGaNAs, or GalnNAs - referred to as

"Guinness", has been demonstrated to increase wavelength. The amount of N needed to

reach 1.3 or even 1.55 μm, however, typically leads to high defect levels that adversely affect device performance and device lifetime. These problems increase dramatically

with the amount of N that is incorporated.

2. Although some researchers believe that a quaternary alloy is formed,

an alternative view still is that the N is incorporated as an impurity or defect state, leading

to gain saturation.

3. The incorporation of N is technologically challenging. There are

problems in reliably incorporating more than 1% N in the active material. Typically, the

material is therefore grown at low temperature, leading to inferior crystal quality, and

heat-treated afterwards. This process, however, cannot completely cure the defects

introduced by the low temperature growth.

Additionally, with regard to U.S. Patent No. 5,719, 895, the following

problems/disadvantages may be encountered when applying the technology to semiconductors:

1. The growth of InAs/GaAs superlattices is extremely difficult with

current technology because of the large lattice mismatch between InAs and GaAs.

2. In highly-strained epitaxial growth, the layers do not grow smoothly,

even well below the critical thickness. Layers can show surface roughness or corrugation

and it is also possible for islanding to occur, resulting in the formation of quantum-dots,

which for purposes of this invention may be regarded as functionally interchangeable

with "quantum islands," with a drastic reduction in the volume of the active region.

3. The theoretical models used to describe the strain usually over¬

simplify the growth process. Other growth modes, such as Stranski-Krastanov, will result

in the formation of quantum dots for layer thickness well below the critical thickness. In

the case where islands or dots are formed, the volume of the active region becomes significantly smaller. This will reduce the maximum achievable gain provided by the

layer. Furthermore, there is likely to be an ensemble of sizes of dots, which results in

broader spectra, with lower peak material gain.

4. Perfectly smooth layers up to the critical thickness have never, as far

as is known, been realized experimentally for the structures proposed in connection with

the present invention. Even in the case of a superlattice structure comprising a repeated

unit of a single InAs layer and a GaAs layer, strain accumulation may result in surface

roughening before the critical thickness is reached. Structural non-uniformities, such as

corrugation, will cause spectral broadening and reduced gain.

5. Growth of these structures is also likely to result in the introduction of

material defects (e.g., dislocations). This will severely reduce the material gain

characteristics and the lifetime of any lasers produced from such material would be short.

One of the most promising, albeit complex, approaches to avoid the problems

outlined above for prior art VCSELs has been to use wafer fusion, in which the active region and

DBRs are grown with separate InP substrates and then bonded together to form the VCSEL.

This may often result in a complicated fabrication process with the associated yield problems and

a high cost of the individual device.

In order to ensure reliability and reproducibility, as well as to overcome the

limitations of the InP/InGaAsP material system, there is continued interest in developing

alternative structures based on GaAs, especially since GaAs-based technology is generally more

advanced than that of InP. However, it is not straightforward to find materials that can be grown

on GaAs with band-gaps that are suitable for 1.3 μm emission. In one approach, quantum dot (QD) structures have been developed. InGaAs QDs

have shown photoluminescence (PL) at 1.3 μm, and a resonant cavity photodiode operating at

1.27 μm has been realized. Recently, an edge-emitting QD laser operating close to 1.3μm has

also been demonstrated. Room temperature (RT) PL at 1.3μm has been observed using strained

GaAsSb quantum wells (QWs) and lasing has been reported in an edge-emitting device at 1.27

μm. An alternative allowing a longer emission wavelength of 1.18μm was implemented in a

GaAs-based VCSEL structure using a single GalnNAs QW, and RT pulsed operation. PL

wavelengths of up to 1.33μm have also been observed in GaAsSb/InGaAs bi-layer QW samples,

with a type-II band-edge alignment. The feasibility of structures with type-II band-edge

alignments for long wavelength devices has been demonstrated as well as the fabrication of edge-

emitting LEDs.

SUMMARY OF THE INVENTION

The present invention overcomes the problems encountered with prior art VCSEL

devices and production methods by providing an active region having a quantum well structure

that can be used in lasers grown on GaAs substrates providing an emission wavelength of 1.0 to

1.6 μm. Such an active region makes use of the combination of strain-compensating barriers

with different band alignments in the quantum wells to achieve a long emission wavelength

while at the same time minimizing the strain in the structure. The invention provides for

structures formed by fabrication of multicomponent alloy layers with a large number of

constituents. The alloy is tailored so that each of these constituents contributes both to the

accumulated strain and to a longer emission absorption wavelength. Devices made in accordance

with the present invention may have utility for both or either of emission and absorption of light, or for light modulation, and may further be by virtue of their physical and optical party adapted

particularly for successful or optimal emission and/or absorption of light having particular

wavelength. In this connection, one may usefully refer to and characterize a device by reference

to a wavelength at which it can efficiently process, i.e. absorb and/or emit, light. The alloy

composition for the quantum well layers is optimized so that the longest possible wavelength is

achieved with the minimum total strain. Additional strain-compensation barrier layers help to

reduce the total strain of the structure further, thereby minimizing the possible formation of

dislocation and other defects that adversely affect device performance and device lifetime. The

most significant constituent in the layer-forming alloys described herein is nitrogen (N), which

leads to a reduction of the bandgap energy (longer wavelength), while at the same time reducing

the lattice constant, thereby reducing the strain. Phosphorus (P) also has this property, while

antimony (Sb) and indium (In) lead to larger lattice constants and therefore cause compressive

strain in layers grown on GaAs. The incorporation of N at levels higher than a few percent is

challenging and potentially leads to poor crystal quality. In combination with the other alloy

constituents, however, it can be used in amounts that are easy to handle from a technological

point of view, while contributing a significant additional amount to the maximum achievable

wavelength.

The active layer structures set forth in the claimed invention herein are

combinations of the basic building blocks as described below and all rely on quantum

confinement. For quantum confinement, layers need to be combined in which the energies of the

valence and conduction bands differ from one layer to another. The simplest such structure, a

quantum well, is shown in Figure 4. A layer with a lower conduction band edge and a higher

valence band edge ~ holes have an inverted energy scale — is sandwiched between layers where the conduction band edge is higher and the valence band edge is lower. The charge carriers,

electrons in the conduction band and holes in the valence band, become trapped in this structure

in quantized levels and recombine very efficiently, emitting light with a wavelength

corresponding to the distance between the lowest conduction band level and the highest valence

band level. Operating the device as a detector, such a quantum well has a very high absoφtion

and therefore a good detectivity. As noted before, a device having such a structure may have

light-processing utility either as a light emitter or a light receptor/detector, or a light modulator,

or any or all of these from time to time depending on intended use.

To increase the efficiency of a quantum-well containing structure, several

quantum wells can be stacked in periodic order, as shown in Figure 5. In the case where the

quantum wells are closely coupled, such an arrangement is called a superlattice and has its own

band structure along the periodic direction, forming an artificially layered material with unique

properties. In the case in which the quantum wells are weakly coupled, the energy levels of the

quantum well are preserved so that the multi quantum well band structure is simply a multiple of

each quantum well.

To reduce further the transition energy, two quantum well layers with different

positions of the valence and conduction band edges can be directly combined, as shown in Figure

6. Such a type-II quantum well has spatially separated regions to trap the electrons and the holes,

leading to a transition between the lowest level in the deepest electron well to the highest level in

the highest hole well. The advantage of a lower transition energy is usually coupled with a

decreasing transition efficiency, since the wave functions for the electrons and the holes are

spatially separated and their overlap is reduced. The exact value of this overlap depends very much on the details of the band structure, and configurations have been demonstrated that show a

very high efficiency similar to the above (type-I) spatially direct quantum wells.

One way of improving the wave function overlap is the symmetric type-II coupled

wells shown in Figure 7, in which a triple-layer structure is formed so that the barrier between

the two electron (hole) wells is relatively low. In this case, the electron (hole) wavefunction has

a high value at the symmetry axis of the structure, where the hole (electron) wavefunction peaks.

This results in a high efficiency of the transition and therefore a good performance of the device.

Typically, the quantum well layers of the structures demonstrated here have a

lattice constant larger than that of the substrate. To reduce the total accumulated strain in the

structures that can lead to defects detrimental to the device performance, the barrier layers on

both sides of the quantum well layers may be made of a material with a smaller lattice constant

than the substrate value. Whereas the layers are then strained with respect to each other, the

average strain of the whole structure is reduced or even vanishes. This design principle is called

strain balancing and is illustrated in Figure 8.

In order to overcome the problems encountered by prior art VCSEL devices and

manufacturing methods the present invention discloses the use of strain-compensated structures

with type-I and type-II band edge alignments. The usage of strain compensation allows multiple

layers to be grown with no degradation of the material quality and therefore offers a higher

degree of freedom in the device design.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 sets forth a schematic design of a generic VCSEL structure with the

variation of the conduction band energy/refractive index/material composition shown to the right. Figure 2 illustrates a typical VCSEL structure showing (a) index guided device,

(b) gain-guided device, (c) oxide confined device.

Figure 3 shows the structure of a generic detector using the same active region

materials as the VCSEL.

Figure 4 depicts a type-I quantum well.

Figure 5 shows a type-I multi quantum well.

Figure 6 shows a type-II quantum well.

Figure 7 shows a type-II symmetric quantum well.

Figure 8 illustrates the principle of strain compensation.

Figure 9 shows band-edge alignment diagram of a type-I strain compensated QW

system designed for emission near 1.3μm.

Figure 10 compares point bandgap energy versus strain for coherently strained

GaPAsSb on GaAs. Negative strain values indicate compressive strain. The right-hand shaded

region is direct bandgap material while the left-hand shaded region is indirect bandgap material.

Figure 11 illustrates an A/B/C/B/A type-I single quantum well utilizing material

system 1 in accordance with the present invention.

Figure 12 illustrates A/B/C/B/A type-I multi quantum wells utilizing material

system 1 in accordance with the present invention.

Figure 13 illustrates an A/B/C/D/B/A type-II single quantum well utilizing

material system 2 in accordance with the present invention.

Figure 14 illustrates A/B/C/D/B/A type-II multi quantum wells utilizing material

system 2 in accordance with the present invention. Figure 15 illustrates an A/B/D/C/B/A type-II single quantum well utilizing

material system 3 in accordance with the present invention.

Figure 16 illustrates A/B/D/C/B/A type-II multi quantum wells utilizing material

system 3 in accordance with the present invention.

Figure 17 illustrates an A/B/D/C/D/B/A type-II single quantum well utilizing

material system 4 in accordance with the present invention.

Figure 18 illustrates A/B/D/C/D/B/A type-II multi quantum wells utilizing

material system 4 in accordance with the present invention.

Figure 19 illustrates an A/B/C/D/C/B/A type-II single quantum well utilizing

material system 5 in accordance with the present invention.

Figure 20 illustrates A B/C/D/C/B/A type-II multi quantum wells utilizing

material system 5 in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Through the use of strained In_wGa₁._wN_xP_yAs_zSb,_.x.y._z/Al_pGa,_.pAs/GaAs

heterostructures, the present invention's material system comprises: 1) Compressively strained

barrier layers with a

type-I band alignment; and 2) Compressively strained quantum wells utilizing multiple In^Ga,.

_wN_xP_yAs_zSb,._x..,__z/In_aGa,._aN_bAs_l__b layers with a type-II band alignment and tensile strained Al_qGa,_

_qN-P-As,...- barrier layers. Both material systems are grown pseudomoφhically on GaAs

substrates. As used herein, "pseudomoφhic" shall mean having a sufficiently low density of

misfit dislocations allowing for the production of lasers having sufficiently long lifetimes. Both

type-I and type-II band-edge alignments are utilized in the present invention. Light emission or absoφtion at wavelengths 1.0 μm to 1.6 μm are achieved by the

present invention by using single or multiple combinations of active materials grown on material

A, with strain compensation material B, with type-I active material C, or with type-II active

materials C and D.

For the type-I active layers:

material system 1 = A-B-(C-B)[n times]-A, n = 1,2,3,....

For the type-II active layers,:

material system 2 = A-B-(C-D-B)[n times]-A, n = 1,2,3,... material system 3 = A-B-(D-C-B)[n times]-A, n = 1,2,3,... material system 4 = A-B-(D-C-D-B)[n times]-A, n = 1,2,3,... material system 5 = A-B-(C-D-C-B)[n times]-A, n = 1,2,3,...

Where the individual layers are: A = Al_pGa,._pAs, 0 < p < 1

B = Al_qGa, JN[_IP_sAs₁._M, 0<q<l;0≤r<0.1;0<s<l

C = In_wGa₁._wN_xP_yAs_zSb₁._x__y._z, 0<w<l;0<x<0.1;0≤y< 0.6; 0 < z < 1 D = In_aGa,._aN_bAs,._b, 01<a≤l;0<b<0.1

The quantum wells ~ layers C and D — are compressively strained, while tensile

strain in the spacer barrier — layer B is used to compensate fully or partially the overall strain in

the active region. The degree of strain compensation affects the total thickness and the number

of quantum wells that can be grown without dislocations. In most embodiments of these multi¬

layer material systems, each layer will lie essentially parallel to the other layers as a result of the

layers being built one atop the other.

Using the model-solid theory, the band edge diagram of the structures disclosed

by the present invention may be constructed as a function of the material composition. As one

illustrative embodiment, Figure 9 shows the conduction band and valence band edge alignments for a particular material system made in accordance with the present invention. The composition

of the barriers is GaP₀₄₂As₀₅₈ and the composition of the well is GaP₀₃₇As_00gSb₀₅₅. The room-

temperature bandgaps of these materials, together with the band discontinuities at the interfaces

are also shown. The strain of the barrier layer is +1.5% (tensile), while for the well layer it is -

3% (compressive). For the barrier and well compositions given above, the transition energy

between the confined electrons and holes within the QW has been calculated as being 0.96eV for

barrier and well widths of 8nm. This result corresponds to a wavelength close to 1.3μm, thus

demonstrating the ability of the present invention to operate in the desired wavelength range.

To demonstrate experimentally the feasibility of these structures, test samples

have been grown by molecular beam epitaxy (MBE) using a chamber equipped with valved

cracking cells allowing for precise layer composition control via digital alloying. Through the

combination of digital alloying and the use of mixed group V elements, it is possible to

practically achieve a layer stoichiometry providing the desired wavelength characteristics (e.g.

1.3μm).

Suφrisingly, it has been found in connection with the present invention that the

inclusion of P and Sb can result in VCSELs having wavelengths > 1.3μm with sufficiently longer

lifetime than conventionally-known devices. This result goes counter to approaches commonly

taken by those ordinarily skilled in the art. Generally, it has been believed that the inclusion of P

and Sb will result in material with an indirect bandgap, particularly in the case where there is

little or no In in the QW. Consequently, such material is believed to have been largely or entirely

overlooked as an element for telecommunications wavelength devices. Additionally, the effects

of strain and composition on direct and indirect bandgaps are not well established. It has been found in connection with the present invention that the inclusion of P and N reduces the material

strain, hence the critical thickness becomes larger. This makes it easier to grow high quality

pseudomoφhic (defect-free) layers of the material, before surface roughening/corrugation

occurs. The use of this material provides a new parameter space, allowing for improved strain-

compensating layers so as to produce high quality epitaxial material. Through the use of the

methods and materials disclosed by the present invention, the composition of the Q W can also be

chosen so that the active material has a direct band-to-band energy transition corresponding to a

wavelength in the vicinity of 1.3μm.

It is the large bandgap bowing parameter of GaPSb that facilitates emission

wavelengths up to and beyond 1.3 μm the (In)GaNPAsSb material system. However,

pseudomoφhic GaPSb on GaAs is an indirect bandgap material for compressive strain levels

below -5% and hence is not suitable as an active material for lasers. To obtain a 0.96 eV (1.3

μm) direct bandgap material at strain levels less than -5%; small amounts of As or InAs must be

added to GaPSb. As an example, the bandgap energy versus strain for GaPAsSb is shown in

Figure 10. In Figure 10, the ternaries GaPAs, GaPSb, and GaAsSb ternaries border the GaPAsSb

quaternary region.

For the region labeled "direct bandgap" (in Figure 10) the lowest energy band to

band transition is between the V point of the conduction band and the T point of the valence

band; these compositions are suitable as active materials for lasers. For the region labeled

indirect bandgap, the lowest energy band to band transition is an indirect transition that occurs

between either the X or the L valley of the conduction band and the T point of the valence band; these compositions are not suitable as active materials for lasers. The bandgap energy values

shown in Figure 10 are calculated at the T point.

The X, L, and F band structure notations refer to separate electron or crystal

momentum values. The term direct transition refers to the movement of an electron between

energy states with the same momentum (i.e., T point to T point), while the term "indirect

transition" refers to a change in both momentum and energy during the transition. Indirect

bandgap materials are not suitable as active materials for lasers because the optical band-to-band

transitions require the mediation of an additional particle in order to conserve momentum; this

greatly reduces the probability of the optical transition occurring.

Because GaP has a large bandgap (> 2 eV) and an indirect bandgap, it is not

obvious that the (In)Ga(N)PAsSb material system is suitable for long wavelength GaAs-based

lasers. The non-obvious attributes of the (In)Ga(N)PAsSb material system that make it suitable

for GaAs-based lasers include, but are not limited to:

1) The large bowing bandgap parameter of GaPSb; see Figure 10, where the

bandgap of the intermediate GaPSb ternary is smaller than that of either binary end point

(GaP or GaSb).

2) The large range of mixed group-V (P, As, and Sb) compositions where

GaPAsSb is a direct bandgap material.

The addition of N and In to these alloys leads to a further reduction in the bandgap

energies and therefore longer operating wavelengths. N seems to incoφorate as a localized state,

causing a strong interaction between this narrow resonant band and the conduction band, thereby

reducing the fundamental band gap of the direct transition. It has been demonstrated that active layers incoφorating N can lase in a VCSEL device with a drastic shift to longer wavelengths as

compared to the N-free alloy for only very small percentages N. Since larger amounts of N are

difficult to incoφorate, for example in InGaAs, it is a challenging task to reach even 1.3 μm for

device applications in these single group V element (As) systems. Generally, the layers need to

be grown at a low growth temperature and annealed afterwards, which results in a high defect

density and structural changes in the alloy, as is evident from a shift in the emission or

absoφtion wavelength. The claimed invention avoids these difficulties by combining N

incoφoration with the structures discussed above, leading to a significant additional shift in the

wavelength while keeping the strain and the defect density low. The increase of the Sb fraction

instead of In with N in these alloys leads to a better N incoφoration and therefore a better layer

quality, since Sb is a better match in terms of growth temperature and sticking coefficient,

especially towards the metal-stable growth regime.

One embodiment of the present invention, denoted herein as system 1 , consists of

an active layer of layer sequence that may be A-B-C-B-A on a substrate close in composition to

GaAs, i.e., comprising GaAs and/or its structural and functional equivlaent in substantial

proportion; where

A = Al_pGa,._pAs, 0 < p < l;

B = Al_qGa^N^As,.,.., 0 < q < 1 ; 0 < r < 0.1; 0 < s < 1;

C = In_wGa,._wN_xP_yAs_zSb₁._x._y._z, 0 < w ≤ l ; 0 < x < 0.1 ; 0 ≤ y < 0.6; 0 < z < 1.

For puφoses of notational clarity, it will be understood that the conceptual layer

sequence designation A-B-C-B-A (as just described, for example) characterizes sequentially-

stacked layers, each layer being adjacent to the next-denoted layer (again following the just- described example, a layer of composition A adjacent to a layer of composition B, itself adjacent

on an opposite side to a layer of composition C, in turn adjacent on its opposite side to a layer of

composition B, which is finally adjacent on its opposite side to a layer of composition A). The

schematic band structure of this layer sequence is shown in Figure 11. For r=s=0 in layer B, the

strain compensation may also be zero.

In another embodiment of the invention, the C-B unit of the active layer may be

repeated, leading to the structure shown schematically in Figure 12. For r=s=0 in the layers B,

the strain compensation may also be substantially zero.

One embodiment of the invention, denoted herein as system 2, consists of an

active layer of layer sequence A-B-C-D-B-A on a substrate close in composition to GaAs; where

A = Al_pGa₁__pAs,0<p<l;

B = Al_qGa^N-P-As,.,.-, 0 < q ≤ 1; 0 < r < 0.1; 0 < s < 1;

C = In_vvGa₁._wN_xP_yAs_zSb₁._x._y._z, 0<w≤l;0<x<0.1;0<y< 0.6; 0 < z < 1 ;

D = In_aGa,._aN_bAs,._b, 0<a<l;0<b<0.1.

The schematic band structure of this layer sequence is shown in Figure 13. For

r=s=0 in layer B, the strain compensation may also be substantially zero.

In another embodiment of the invention, the C-D-B unit of the active layer may be

repeated, leading to the structure shown schematically in Figure 14. For r=s=0 in the layers B,

the strain compensation may also be zero.

One embodiment of the invention, denoted herein as system 3, consists of an

active layer of layer sequence A-B-D-C-B-A on a substrate close in composition to GaAs; where

A = Al_pGa,__pAs,0<p<l; B = A-_qGa^N-P-As,^, 0 < q < 1; 0 < r < 0.1; 0 < s < 1;

C = In_wGa,._wN_xP_yAs_zSb,._x._y__z, 0 < w < 1 ; 0 < x < 0.1 ; 0 < y < 0.6; 0 < z < 1 ;

D = In_aGa,._aN_bAs,__b, 0<a≤l;0<b<0.1.

The schematic band structure of this layer sequence is shown in Figure 15. For

r=s=0 in layer B, the strain compensation may also be zero.

In another embodiment of the invention, the D-C-B unit of the active layer may be

repeated, leading to the structure shown schematically in Figure 16. For =S=0 in the layers B,

the strain compensation may also be substantially zero.

One embodiment of the invention, denoted herein as system 4, consists of an

active layer of layer sequence A-B-D-C-D-B-A on a substrate close in composition to GaAs;

where

A = Al_pGa,._pAs,0<p<l;

B = Al_qGa_LqN^As,.^ 0 < q < 1; 0 < r < 0.1; 0 < s < 1;

C = Iι\_vGa,._wN_xP_yAs_zSb,._x._y__z, 0<w<l;0<x<0.1;0≤y< 0.6; 0 < z < 1 ;

D = In_aGa,._aN_bAs,._b, 0 < a < 1; 0 < b < 0.1.

The schematic band structure of this layer sequence is shown in Figure 17. For

r=s=0 in layer B, the strain compensation may also be substantially zero.

In another embodiment of the present invention, the D-C-D-B unit of the active

layer may be repeated, leading to the structure shown schematically in Figure 18. For r=s=0 in

the layers B, the strain compensation may also be substantially zero.

One embodiment of the present invention, denoted system 5, consists of an active

layer of layer sequence A-B-C-D-C-B-A on a substrate close in composition to GaAs; where A = Al_pGa,__pAs, 0 < p < l;

B = Al_qϋa^HP-As,_.,_.-, 0 < q < 1; 0 < r < 0.1; 0 < s < 1 ;

C - In_wGa,._wN_xP_yAs_zSb₁._x__y._z, 0 < w < 1 ; 0 < x < 0.1 ; 0 < y < 0.6; 0 < z < 1 ;

D = In_aGa,._aN_bAs,._b, 0 < a ≤ l ; 0 < b < 0.1.

The schematic band structure of this layer sequence is shown in Figure 19. For

r=s=0 in layer B, the strain compensation may also be substantially zero.

In another embodiment of the present invention, the C-D-C-B unit of the active

layer may be repeated, leading to the structure shown schematically in Figure 20. For r=s=0 in

the layers B, the strain compensation may also be substantially zero.

It will be evident to those of ordinary skill in the art that the above-described

modes and embodiments of the present invention, while they disclose useful aspects of the

present invention and its advantages, are illustrative and exemplary only, and do not describe or

delimit the spirit and scope of the present invention, which are limited only by the claims that

follow below. All references referred to hereinabove are incoφorated by reference herein in full.

Claims

We claim as our invention:

1. A light processing device comprising: (a) a substrate and (b) an active region, wherein

said substrate comprises a semiconductor material having a lattice constant, and wherein said

active region comprises a plurality of layers including: (i) at least one pseudomoφhic light

processing layer comprising In_wGa,__vvN_xP_yAs_zSb,.₎.._y._z (0<w<l,0<x<0.1,0<y< 0.6, 0 < z

< 1) and; at least one pseudomoφhic barrier layer comprising Al_qGa_l ,N_rP_sAs_tSb,_.r._s._t (0 < q <

l,0≤r<0.1,0<s≤l,0<t<l), and wherein each of said plurality of layers in said active

region has a composition different from the composition of any adjacent layer.

2. The device of claim 1, wherein light processing comprises processing selected from: (a)

emitting light; (b) receiving light; (c) sensing light; and (d) modulating light.

3. The device of claim 2, wherein said active region further comprises (iii) at least one

pseudomoφhic light processing layer comprising In_aGa,__aN_bAs,._b (0 < a < 1, 0 < b < 0.1).

4. The device of claim 3, wherein each of said plurality of layers in said active region lies

substantially parallel to a common plane.

5. The device of claim 2, wherein said active region comprises a sequence of layers A-B-C-

B-A, wherein layer A comprises Al_pGa,._pAs (0 < p < 1), layer B comprises Al_qGa,_

_qN.P.As.Sb,...^, (0 < q < 1, 0 < r < 0.1, 0 < s < 1, 0 < t < 1), and layer C comprises Ir^Ga,.

_wN_xP_yAs_zSb,_.x.y.z (0 ≤w≤ l, 0 <x<0.1, 0 ≤y < 0.6, 0 < z < 1), and wherein the band

structure formed between layers B and C has a type-I band-edge alignment.

6. The device of claim 2, wherein said active region comprises a sequence of layers A-B-(C-

B)χn-A, wherein layer A comprises Al_pGa,._pAs (0 <p < 1), layer B comprises Al_qGa,_.

_qN_P_sAs_tSb,_.r.s._t (0 < q < 1, 0 < r < 0.1, 0 < s < 1, 0 < t < 1), and layer C comprises Ir^Ga,_.

_wN_xP_yAs_zSb,__x._y__z (0<w≤l,0<x<0.1,0<y< 0.6, 0 < z < 1), and wherein the sequence of

layers (C-B) is repeated in adjacent succession n times, n being an integer number larger than

1, and wherein the band structure formed between layers B and C has a type-I band-edge

alignment.

7. The device of claim 2, wherein said active region comprises a sequence of layers A-B-C-

D-B-A, wherein layer A comprises Al_pGa,__pAs (0 <p < 1), layer B comprises Al_qGa,_

_qN-P_sAs.Sb,._^.. (0 <q < l, 0 <r≤ 0.1, 0 ≤ s < l, 0 <t < 1), layer C comprises In_^Ga,.

_wN_xP_yAs_zSb,._x._y._z (0<w≤ l,0<x<0.1,0<y< 0.6, 0 < z < 1), layer D comprises In_aGa,.

_aN_bAs,_.b (0 < a < 1, 0 < b < 0.1), and wherein the band structure formed between layers B and

C has a type-I band-edge alignment, and wherein the band structure formed between layers B

and D has a type-I band-edge alignment, and wherein the band structure formed between

layers C and D has a type-II band-edge alignment.

8. The device of claim 2, wherein said active region comprises a sequence of layers A-B-(C-

D-B)χn-A, wherein layer A comprises Al_pGa,._pAs (0 <p < 1), layer B comprises Al_qGa,_

_qN-P-AS_tSb,...-., (0 <q < l, 0 <r< 0.1, 0 ≤ s < l, 0 <t < 1), layer C comprises Ir^Ga,.

_wN_xP_yAs_zSb,._x._y._z (0<w< l,0<x<0.1,0≤y< 0.6, 0 < z < 1), layer D comprises In_aGa,.

_aN_bAs,_.b (0 < a < 1, 0 < b < 0.1), and wherein the sequence of layers (C-D-B) is repeated in

adjacent succession n times, n being an integer number larger than 1, and wherein the band

structure formed between layers B and C has a type-I band-edge alignment, the band structure formed between layers B and D has a type-I band-edge alignment, and the band

structure formed between layers C and D has a type-II band-edge alignment.

9. The device of claim 2, wherein said active region comprises a sequence of layers A-B-D-

C-B-A, wherein layer A comprises Al_pGa,__pAs (0 <p < 1), layer B comprises Al_qGa,_

_qN_rP_sAs_tSb,._r.-._t (0 <q < l, 0 <r< 0.1, 0 ≤ s < l, 0 <t < 1), layer C comprises In_wGa,_.

_wN_xP_yAs_zSb,._x._y._z (0<w≤ l,0<x<0.1,0<y< 0.6, 0 < z < 1), layer D comprises In_aGa,_.

_aN_bAs,._b (0 < a < 1, 0 < b < 0.1), and wherein the band structure formed between layers B and

C has a type-I band-edge alignment, the band structure formed between layers B and D has a

type-I band-edge alignment, and the band structure formed between layers C and D has a

type-II band-edge alignment.

10. The device of claim 2, wherein said active region comprises a sequence of layers A-B-(D-

C-B)xn-A, wherein layer A comprises Al_pGa,._pAs (0 <p < 1), layer B comprises Al_qGa,_.

_qN-P-As.Sb,...,., (0 <q < l, 0 <r≤0.1, 0 ≤ s < l, 0 <t < 1), layer C comprises Ir^Ga,_.

_wN_xP_yAs_zSb,._x._y._z (0<w< l,0<x<0.1,0<y< 0.6, 0 < z < 1), layer D comprises In_aGa,_.

_aN_bAs,__b (0 < a < 1, 0 < b < 0.1), and wherein the sequence of layers (D-C-B) is repeated in

adjacent succession n times, n being an integer number larger than 1, the band structure

formed between layers B and C having a type-I band-edge alignment, the band structure

formed between layers B and D having a type-I band-edge alignment and the band structure

formed between layers C and D having a type-II band-edge alignment.

11. The device of claim 2, wherein said active region comprises a sequence of layers A-B-D-

C-D-B-A, wherein layer A comprises Al_pGa,._pAs (0 <p < 1), layer B comprises Al_qGa,_ _qN_rP_sAs_tSb,_.,_s.t (0 <q < l, 0 < r< 0.1, 0 ≤ s < l, 0 < t < 1), layer C comprises Ir^Ga,_.

^P_yAs.Sb,_.^., (0<w≤ l,0<x<0.1,0≤y< 0.6, 0 < z < 1), layer D comprises In_aGa,_.

type-II band-edge alignment.

12. The device of claim 2, wherein said active region comprises a sequence of layers

A-B-(D-C-D-B)χn-A, wherein layer A comprises Al_pGa,._pAs (0 < p < 1), layer B comprises

Al_qCa^_qN-P-A^Sb,.^, (0<q≤l,0<r<0.1,0<s<l,0<t<l), layer C comprises In ja,.

_wN_xP_yAs_zSb,._x._y._z (0<w< l,0<x<0.1,0<y< 0.6, 0 < z < 1), layer D comprises In_aGa,_

_aN_bAs,__b (0 < a < 1, 0 < b < 0.1), and wherein the sequence of layers (D-C-D-B) is repeated n

times in adjacent succession, n being an integer number larger than 1, and wherein the band

structure formed between layers B and C has a type-I band-edge alignment, the band

structure formed between layers B and D has a type-I band-edge alignment, and the band

structure formed between layers C and D has a type-II band-edge alignment.

13. The device of claim 2, wherein said active region comprises a sequence of layers A-B-(C-

D-C-B)χn-A, wherein layer A comprises Al_pGa,._pAs (0 < p < 1), layer B comprises Al_qGa,_

_qN_rP_sAs_tSb,_.r.s.t (0 <q< l, 0 <r< 0.1, 0 ≤ s < 1, 0 <t< 1), layer C comprises _wN_xP_yAs_zSb,_.x.y.z (0<w< l,0<x<0.1,0≤y< 0.6, 0 < z < 1), layer D comprises In_aGa,_

_aN_bAs,__b (0 < a < 1, 0 < b < 0.1), and wherein the sequence of layers (C-D-C-B) is repeated in

structure formed between layers B and C has a type-I band-edge alignment, the band structure formed between layers B and D has a type-I band-edge alignment and the band

structure formed between layers C and D has a type-II band-edge alignment.

14. The device of claim 2 or 3, wherein at least one of said plurality of layers comprising said

active region acquires quantum dots during a growth process in which said at least one layer

is formed.

15. The device of claim 2, wherein the wavelength of light that may be processed by the

device at a temperature of about 300 K is at least about 1150 nm.

16. The device of claim 2, wherein the wavelength of light that may be processed by the

device is at about least 1150 nm at temperatures greater than about 300 K.

17. The device of claim 2, wherein the wavelength of light that may be processed by the

device is at least about 1150 nm at temperatures less than about 300 K.

18. The device of claim 2, wherein said substrate comprises GaAs having a lattice constant.

19. The device of claim 2, wherein said substrate comprises GaAs in combination with an

additional substrate constituent selected from the group consisting of: Al, In, and a dopant.

20. The device of claim 2, further comprising: (c) a first conductive layer having a first

conductivity type and placed in electrical contact with said active region; (d) a second

conductive layer having a second conductivity type and placed in electrical contact with said

active region; and (e) an electrical connection to said active region, wherein said electrical

connection has the capability of processing electrical current, and wherein said processing comprises processing selected from the group comprising: (i) supplying electrical current to

said active region; and (ii) receiving electrical current from said active region.

21. The device of claim 20, wherein a bandgap associate with said first and second

conductive layers is larger than that of any bandgap associated with said sequence of layers in

said active region.

22. The device of claim 20, further comprising a semiconductor-air interface, wherein said

semiconductor interface is adapted for light processing.

23. The device of claim 22, wherein said semiconductor interface is formed by a processing

step, selected from the group consisting of etching and cleaving, to form a cavity normal to a

plane substantially parallel to each of said plurality of layers.

24. The device of claim 20, further comprising: (f) a grating layer disposed above said

second conductive layer, wherein said grating layer contains lines extending over at least a

portion of said active region, and wherein said grating layer defines an optical cavity having

an optical resonance, with the resonance being associated with a resonance wavelength

corresponding to a resonance energy, and wherein the resonance wavelength in microns as

measured in a vacuum is about equal to 1.24 divided by the resonance energy measured in

electron volts.

25. The device of claim 24, wherein the grating lines are shifted by n multiplied by one

quarter of the resonance wavelength, wherein n is an integer greater than or equal to 1 ,

whereby a phase-shifted grating is formed.

26. The device of claim 20, further comprising: (f) a bottom mirror placed underneath said

active region; (g) and a top mirror placed above said active region, said mirrors defining an

optical cavity resonance associated with a resonance energy and a corresponding resonance

wavelength, and wherein the resonance wavelength in microns as measured in a vacuum is

about 1.24 divided by the resonance energy measured in electron volts.

27. The device of claim 26, wherein said bottom mirror comprises a plurality of higher-

refractive-index and lower-refractive-index layers in alternating adjacent disposition.

28. The device of claim 27, wherein at least one of said plurality of bottom mirror layers of

lower-refractive-index comprises oxidized material.

29. The device of claim 26, wherein said top mirror comprises comprises a plurality of

higher-refractive-index and lower-refractive-index layers in alternating adjacent disposition.

30. The device of claim 29, wherein each said lower-refractive-index layer is selected from

the group consisting of: oxidized material; low-index dielectric material; and low index

semiconductor material.

31. The device of claim 29, wherein each said higher-refractive-index layer is chosen from

the group consisting of: high-index dielectric material; and high index semiconductor

material.

32. The device of claim 20, further comprising: (f) a two-region aperture disposed between

said top mirror and said active region.

33. The device of claim 32, wherein a first region of said two-region aperture has a relatively

low electrical resistance and a second region of said two-region aperture has an electrical

resistance that is higher than the resistance of said first region of said aperture.

34. The device of claim 32, wherein a first region of said two-region aperture has a refractive

index that is higher than than the refractive index of a second region of said two-region

aperture.

35. The device of claim 32, wherein said two-region aperture comprises oxidized material

and wherein a second region of said aperture is more highly-oxidized than a first region of

said aperture.

36. The device of claim 32, wherein said two-region aperture is formed by etching a pillar

structure.

37. The device of claim 32, wherein said device comprises a resonant cavity photodetector

(RCPD).

38. The device of claim 32, wherein said device comprises a resonant cavity light emitting

diode (RCLED).

39. The device of claim 32, wherein said device comprises a vertical cavity surface emitting

laser (VCSEL).

40. The device of claim 32, wherein said device comprises a vertical cavity surface emitting

laser for fiber optic data communications.

41. The device of claim 40, wherein said device has an emission wavelength between about

1260 nm and about 1360 nm.

42. The device of claim 40, wherein said device has an emission wavelength between about

1360 nm and about 1460 nm.

43. The device of claim 40, wherein said device has an emission wavelength between about

1460 nm and about 1610 nm.

44. The device of claim 20, wherein said device comprises an optical modulator.