NASA Contractor Report 4252 

Approaches Toward a Blue 

Semiconductor Laser 

I. Ladany 

Planning Researcb Corporation 

Aerospace TecbnoZogies Division 

Hampton, Virginia 

Prepared for 

Langley Research Center 

under Contract NAS1-18000 

National Aeronautics and 

Space Administration 

Office of Management 

Scientific and Technical 

Information Division 

1989 
ABSTRACT 

This paper surveys approaches which can be used to obtain 

semiconductor diode laser action in the blue region of the 

spectrum. Included is a discussion of diode lasers in general, 

and a review of the current status of "visible" emitters, 

presently limited to about 670 nm. Various methods are discussed 

for shifting laser emission toward shorter wavelengths, including 

the use of 11-VI materials, the increase in the bandgap of 111-V 

materials by additions of low atomic number elements, for example 

nitrogen; changing the band structure from indirect to direct by 

incorporating interstitial atoms or by constructing certain 

superlattices. 

including avalanche breakdown, Langmuir-Blodgett diodes, heterostructures, carrier accumulation, and Berglund diodes. A 

discussion is given of the prospects of inventing new multinary 

semiconducting materials, and a number of novel materials 

described in the literature are tabulated. 

available through the development of quantum wells and superlattices are discussed, including resonant tunneling, which may 

have applications to materials not having pn-junction capability, 

and the synthesis of arbitrary bandgap materials through multiple 

quantum wells. Also included is a review of the current status 

of some materials often considered in discussions of semiconductor 

lasers or emitters (Sic, ZnO, ZnS, ZnSe, GaN and InN, Diamond, 

CdS) . 

Non-pn-junction injection methods are surveyed, 

New approaches 

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iii 
TABLE OF CONTENTS 

SECTION 

LIST OF ILLUSTRATIONS 

S.Y ................................................ 1 

I . INTRODUCTION ........................................... 4 

I1 . THE DIODE LASER ........................................ 7 

A . Basic structure of the diode laser .................. 7 

B . State of the art in diode lasers ................... 10 

I11 . INJECTION MECHANISMS .................................. 13 

A . P-N Junction ....................................... 14 

B . Schottky Barrier ................................... 16 

C . MIS and MOS Devices ................................ 17 

D . Langmuir-Blodgett or Tunneling MIS Devices ......... 17 

E . Heterojunction Barriers ............................ 19 

F . Electron Beam Injection ............................ 19 

IV . 

V . 

VI . 

VI1 . 

G . AC Electroluminescence ............................. 20 

H . Carrier Accumulation ............................... 21 

I . Spark-gap Excitation ............................... 21 

J . Avalanche Multiplication ........................... 22 

K . Berglund Effect .................................... 22 

LASER MATERIALS ....................................... 23 

A . Modifications of Existing Materials ................ 23 

B . Prospects for the Synthesis of New Materials ....... 28 

C . A Table of Materials ............................... 31 

HETEROSTRUCTURES ...................................... 36 

FREQUENCY MULTIPLICATION .... .......................... 38 

ELECTRON BEAM PUMPED LASERS ........................... 40 

V 

PRECEDILG PAGE BLANK NOT FILMED 
VIII. QUANTUM WELLS AND SUPERLATTICES.......................41 

IX. SPECIAL TOPICS........................................46 

A. SiC................................................46 

B. ZnO................................................47 

C. ZnS................................................48 

D. ZnSe........................ ....................... 48 

E. GaN, InN...........................................50 

F. Diamond............................................51 

G. CdS................................................52 

X. CONCLUSIONS...........................................54 

XI. ACKNOWLEDGMENTS........... ............................ 55 

XII. REFERENCES............................................56 

vi 
LIST OF ILLUSTRATIONS 

Figure Page 

1 

2 

3 

4 

8 

9 

Basic Structure of a Diode Laser...........................69 

Power Curve of a Typical InGaAsP Laser.....................70 

Direct and Indirect Bandgaps in AlGaAs.....................71 

Graded-Index Separate-Confinement Quantum-Well 

Laser Structure ............................................ 72 

Structure and Band Diagrams of a p-n Junction. ............. 73 

Electron and Hole Injection in a p-n Junction .............. 74 

Metal-Semiconductor Barrier, a) Without and b) With 

Forward Bias .............................................. 75 

Mechanism of the Tunneling MIS Diode... .................... 76 

AlGaAs/GaAs Heterointerface, a) Without and 

b) With Forward Bias ....................................... 77 

10 Structure of a Frequency-Conversion Laser..................78 

11 First Two Energy Levels in the a) Infinite, and 

b) Finite Potential Well...................................79 

12 Density of States Function in the Conventional 

(3 Dimensional) Structure, and in a Single Quantum 

Well (2 Dimensional) Structure.. ........................... 80 

13 The Mechanism of Resonant Tunneling. Application of 

the Correct Bias Lines up the Fermi Level with the 

First Allowed Level in the Quantum Well....................81 

vi i 
SUMMARY 

Short wavelength lasers, especially in the blue spectral 

region, are of interest wherever high power density is to be 

delivered to a surface, for example in laser recording devices. 

This report attempts to survey various approaches available for 

making such a device. Beginning with a review of the main 

properties of diode lasers which make them attractive in such 

applications, the discussion covers the state of the art in diode 

lasers, currently limited to about 670 nm. This limitation arises 

out of the lack of semiconductor materials having the requisite 

bandgap while being capable of both p- and n-type doping to high 

levels. Certain 11-VI materials, for example ZnO, ZnSe, and CdS, 

have the required bandgap, and have been made to lase using 

electron beams and other methods; however, they cannot be made to 

have efficient p-n junctions. A section of this report therefore 

deals with alternate injection mechanisms which might be 

applicable to such materials. 

be promising, none are as efficient as p-n junctions. 

Although some of these are found to 

Consideration is next given to modifications which might 

be imposed on existing laser materials to make their emission 

reach the desired wavelength. These include increasing the 

bandgap by additions of low atomic number elements, e.g.,nitrogen 

to 111-V materials, such as GaAlAs, the possibility of making 

ambipolar 11-VI materials through use of non-equilibrium growth 

methods, and the change of bandstructure from indirect to direct, 

lby for example constructing superlattices using GaP and AlGaP, or 

PRECEDING PAGE BLANK NOT FILMED 
by the incorporation of interstitial atoms into a suitable 

crystal, also typically Gap. 

The next section examines the possibility of inventing (or 

discovering) new multinary compound semiconductors, and concludes 

that the number of possible direct bandgap compounds with bandgaps 

in the 2.5-3.0 eV region is very large. 

Evidently, this field will prove to be extremely fruitful in 

the future, although it will also require a significant effort. 

A more direct approach to the possibility of developing new 

materials is to consider some of those which have already been 

studied, but which have so far not been successful. A table of 

such materials, binaries, ternaries and a few quaternaries, 

provides a listing some of the pertinent data, such as bandgaps 

and crystal structures. Examples are CdGa2S4, CuGaS2, and 

ZnSel-xTex. 

Another approach consists of constructing devices incorporating p- and n-type heterostructures between dissimilar 

materials. This process has never been successful in the past, 

but new insights obtained in recent years in the field of strained 

layer structures combined with new methods of growth, could lead 

to ways of overcoming the lack of p-doped materials which has 

limited the usefulness of 11-VI compounds. 

Two sections, one on frequency multiplication and one on 

electron beam pumped lasers, are included in order to cover these 

very active fields, although neither of them is attractive as a 

substitute for diode lasers. 

Quantum well lasers and superlattice structures are surveyed, 

and it is shown that they offer improved performance and the 
possibility of novel developments, such as new injecting methods 

and the synthesis of desired bandgap materials. 

The final section of the report consists of a survey of the 

present state of development in a number of materials sometimes 

thought of as promising light emitters. These include Sic, ZnO, 

CdS, ZnSe, GaN and InN, and Diamond. 

3 
Section I. 

INTRODUCTION 

The possibility of developing a shorter wavelength diode 

laser is attracting the attention of a number of laboratories 

throughout the world. Besides applications which particularly 

require such wavelengths (strong or weak absorption in specific 

regions), the advantage of shorter wavelengths resides in the 

greater power density which such a laser can deliver into an 

optical system. 

The main objective of the present study is to provide a 

survey of devices which have the potential to generate such short 

wavelength laser radiation in a solid-state diode structure. The 

ideal device fitting this description would be a laser similar to 

the well-known AlGaAs heterostructure type, but emitting in the 

blue region of the spectrum. Such a device would operate at room 

temperature at a current of less than 100 mA, with a bias of 

about 2.5 volts. Although this device cannot now be made, 

continued efforts in this direction are likely to lead to eventual 

success. It is hoped that the present report will provide an 

outline of the various ways this goal can be pursued. 

The principal reason for NASA's interests in these short 

wavelength lasers is their use in recording applications'. 

Currently, studies of erasable optical recording seem to be 

aimed at three different approaches: magneto-optical (MO), dyepolymer (DP), and phase change (PC). Of these, MO appears to 

be the most advanced, but the other two are also being pursued 

intensively. 

4 
In magneto-optical recording, the laser heats a small region 

on the disc, and a coil is used to align the direction of 

magnetization in this region. For read out, a polarized light 

beam is needed to sense the state of polarization. Thus, a higher 

power source of light which need not be polarized, and a lower 

power source which must be polarized are required for the 

operation, and this is clearly available from a single laser whose 

power can be modulated by varying the applied current. For the 

PC method, in which the reflectivity of a suitable material is 

changed by taking it from the crystalline to the amorphouse state, 

the light source is used to generate the required amount of heat 

only. Similarly, in the DP method, a plastic film is distorted 

through the application of heat, again imposing no requirements 

on the polarization of the light source. It must therefore be 

allowed, that there may be ways of achieving rapid optical 

recording using non-lasing light sources, provided they are 

intense enough and have a short enough wavelength2. Nevertheless, a laser beam has a much higher radiance, and is therefore 

more efficiently focused into a spot. In this review we will, 

therefore, be mostly interested in direct bandgap materials 

capable of generating stimulated emission in semiconductors, 

although occasionally there will be mention of indirect materials 

if they have some especially interesting properties. 

Excluded from this survey are large classes of lasers, such 

as gas lasers, ion lasers, dye lasers, excimer lasers, and flashlamp pumped solid state lasers, the exclusion being due entirely 

to their large size and low efficiency. 

that the distinction is not well defined, and that in the future 

However, it is recognized 

5 
some heretofore excluded lasers may have to be considered. Partly 

because of their position at the border between these two classes, 

we include electron beam pumped lasers and diode-pumped frequencyconversion lasers in this review. 

Using the AlGaAs laser as a model, it is possible to state 

the requirements for developing a useful laser in a new wavelength 

regime. Mainly, this will require material with a direct bandgap 

of the correct magnitude, the ability to make p-n junctions, and 

the ability to achieve high doping levels with high electrical 

conductivity. 

Looking at 111-V compounds used in all currently successful 

structures, it appears that there are none capable of satisfying 

all of these requirements. Thus the only 111-V material known 

to have a large enough direct bandgap is GaN, although it lacks a 

number of other properties from the list, particularly that of 

forming p-n junctions. The prospects for this as well as a number 

of other materials, especially compounds of the 11-VI composition, 

would be much improved if other injection processes not based on 

p-n junctions could be used. It seemed useful, therefore, to 

include a review of various non- p-n junction injection mechanisms 

described in the literature. 

A part of this review deals with possible approaches toward 

modifying existing lasers in suitable directions, for example by 

modifying 111-V materials through additions of nitrogen. A 

section is devoted to a listing of relatively new materials 

discussed in the literature, which show some promise of yielding 

shorter wavelength lasers. 

The last class of devices to be considered are based on 

6 
quantum wells (QW), superlattices, and their prospects in the 

development of short wavelength lasers. The concluding section 

of the review deals with a number of materials which were at 

one time considered promising, and which for various reasons have 

not yet lived up to their potential. This includes diamond, GaN, 

InN, Sic, and a series of compounds from columns 11-VI: ZnSe, 

ZnS, ZnO, and CdS. 

Section 11. 

THE DIODE LASER 

A laser light source has certain unique properties: 

emission in the form of a fairly tight, linearly polarized beam 

with a narrow range of wavelengths. Further, the phase front of 

the beam along the axis of propagation is approximately planar, 

and the beam intensity distribution is Gaussian and lends itself 

to efficient focusing into a small area. Furthermore, the laser 

as a source of light is very efficient. Thus, overall power 

efficiencies (light power out divided by electrical power in) for 

diode lasers can exceed 50 percent, a value unmatched by any other 

light source. It is for these reasons that one wishes to apply 

laser technology in various areas, including that contemplated in 

this work, the recording of data. 

A. BASIC STRUCTURE OF THE DIODE LASER 

In order to construct a laser it is necessary to produce 

an excited state with an inverted population, and this population 

must be capable of returning to the equilibrium state through 

7 
giving up energy in the form of light. The process of light 

generation in semiconductors is called radiative recombination. 

The excited state of electrons or holes can be produced in 

different ways, for example by optical pumping (flash lamp), by 

bombardment with an electron beam, or by the injection of minority 

carriers by means of a p-n junction. Whatever the method, the 

excited electrons must also be placed into an optical resonator 

which selects some particular mode into which optical energy 

will be continuously fed by the recombining electrons. 

A simple structure of a diode laser is shown in figure 1. 

The active layer, where recombination takes place, is GaAs, and 

this layer is surrounded on two sides by layers of AlGaAs. These 

two layers have a lower refractive index than GaAs, and a higher 

bandgap - these characteristics serving to contain both electrons 

and photons within the GaAs slab. 

by cleaving the ends of the crystal to give two perfectly flat 

and parallel surfaces, having a reflectivity of about 30 percent. 

The lateral boundaries in this structure are the sides of the 

slab, which is not a desirable arrangement for various reasons. 

Improvements are effected by a) restricting the current flow to a 

narrow region in the middle of the slab through use of a stripe 

contact, b) by reducing the size of the active region in the 

lateral direction by etching out a mesa, c) by building in 

refractive index steps in the lateral direction, and d) by introducing back-biased junctions in the lateral direction in order 

to restrict the current flow exclusively to the center portion. 

The optical resonator is formed 

Any and sometimes all of these techniques are used in the 

same device, and their effect is to concentrate the current, the 

8 
injected carriers, and the generated photons into the smallest 

possible volume. The exact form of these lateral modifications 

is not important for the present purposes; it is sufficient to 

assume that one can always introduce some form of lateral 

confinement by the proper application of technology. 

A light-output-power versus current-input curve of a typical 

laser with some lateral confinement is shown in figure 2. The 

value of the current where the light output begins to increase 

rapidly is called the threshold current, and that value divided 

by the active area is the threshold current density. The threshold current shown is about 20 mA; some commercially available 

units have a threshold current of a few mA. If one knows or if 

one can estimate the size of the active area, the threshold 

current density provides a way of comparing the quality of different lasers. For sustained performance, this value should be well 

below 2000 A/cm2; in the better lasers it is below 1000 A/cm2, 

and in the latest quantum well lasers it can be less than 250 A/ 

cm2. The lower this value, the more efficient the laser and 

the more power can be generated with it (although this may 

require that the geometry of the structure be designed for that 

purpose). 

The quantum well laser is a radical modification of the 

structure discussed so far, based on making the active layer 

extremely thin (say 5 nm). These lasers, and the related superlattice lasers, will be discussed further below. 

9 
B. STATE OF THE ART IN DIODE LASERS 

The present section offers a review of the state of the art 

in short wavelength injection lasers. As will be seen, these 

lasers are rapidly approaching commercial exploitation, and are 

likely to displace He-Ne lasers in many applications. However, 

their wavelength is in the vicinity of 670 nm. 

1. AlGaAs Lasers 

The emission wavelength of an AlGaAs laser can be changed by 

increasing the ratio of Alto Ga. This is a consequence of the 

fact that the bandgap of the alloy increases with increasing Al. 

In order to maintain the bandgap and index steps at the heterointerfaces, it is also necessary to increase the A1 concentration 

in the cladding layers. The limiting aluminum concentration in 

the active layer is that for which the material becomes indirect’, 

which, according to figure 3, corresponds to an A1 mole fraction 

of 0.38 and a bandgap of 1.96 eV. However, various difficulties 

arise as the bandgap approaches this value, such as an increase ir 

threshold; and what is more serious, the operating life also 

appears to decline with increasing aluminum content. It seems 

that the shortest wavelength achievable using DH lasers in this 

system without any effect on device design or performance is near 

720 nm. 

Progress in AlGaAs has been achieved by using quantum 

wells (see Section VI11 below). The first step in the realization 

of this structure is to make the active layer extremely thin, 

typically 3-5 nm. This already requires a profound change in the 

crystal growth apparatus, since liquid-phase-epitaxy (LPE) is not 

10 
capable of providing the layer flatness and uniformity required. 

Rather, one uses organo-metallic vapor phase epitaxy (OMVPE) or 

molecular beam epitaxy (MBE). Another modification is made in 

the structure outside the active region, which in these devices 

consists of a graded index region serving the purpose of further 

confining the radiation to the vicinity of the active region , 

while discouraging higher-order mode excitation (see figure 4). 

The single, very thin layer in a quantum well laser can also be 

replaced by a stack of alternating high and low bandgap layers, 

forming the multiple quantum well laser, further discussed in 

Section VIII. Using such graded index quantum well lasers, a 

threshold current density of approximately 1.1 kA/cm2 was 

obtained at 680 nm, but no data was supplied concerning life5. I 

4 

2. InGaAsP ~ 

The active layer here is composed of the same elements as 

the long wavelength (1.3 and 1.5 um) communications lasers. 

However, the proportions are different, and the lattice constant 

of InP does not match the lattice constant of InGaAsP having the 

bandgap corresponding to this wavelength. In order to have a 

strain-free system, the cladding has to be InGaP, which is grown 

on a suitable composition of GaAsP6. A threshold current 

density of 4.5 kA/cm2 and a threshold current of 56 mA were 

obtained at 671 nm. 

3. AlGaInP 

This material is also a promising candidate for short wavelengths, and can be taken as a derivative of the InGaP system. 

1 

At a wavelength of 662 nm, a threshold current of 120 mA and a I 

11 
threshold current density of 6.7 kA/cm2 were obtained7. 

4. Quantum Well InGaP/AlGaInP 

This laser is composed of multiple InGaP wells and AlGaInP 

barrier layers8. The threshold current and threshold current 

density are 70 mA and 3.5 kA/cm2 respectively, at 668 nm. 

Another recent result, in the same system, is a threshold current 

of 55 mA or 4.4 kA/cm2 at 646 nm9. Work with optically 

pumped lasers suggests that the AlGaInP QW system could be pushed 

to a wavelength as short as 625 nmlO. 

A summary of these results is presented in the table below: 

TABLE 1: Recently Published Results 

for Short Wavelength Diode Lasers. 

System Wavelength (nm) 

AlGaAs QW 1.1 

InGaAsP 4.5 

InGaAlP 6.7 

InGaP/InGaAlP QW 3.5 

4.4 

680 

671 

662 

668 

646 

The best threshold results seem to have been achieved in the 

AlGaAs system. Nevertheless, probably because of reduced life, 

visible lasers now becoming available commercially are based on 

InGaAlP. 

Table 2 shows properties of those lasers advertised by NEC 

and Toshiba: 

12 
Table 2: Visible Diode Lasers 

Available Commercially. 

composition InGaAlP 

threshold current 76-100 mA 

rated power 

wavelength 

life 

3 mW 

670 nm 

>2500 h @ 3 mW, 50 C 

Section 111. 

INJECTION MECWWISMS 

Carrier injection in typical semiconductor lasers occurs 

via p-n junctions. This method appears to be the most efficient 

known, in the sense of requiring the smallest power consumption 

for its operation". However, there are presently very few 

semiconductors which are capable of supporting p-n junctions while 

possessing the required bandgap value for short wavelength 

emission. This is thought to be a consequence of the low enthalpy 

for vacancy formation12, these vacancies often acting as 

electrically active centers which can compensate the usual doping 

impurities. The general tendency for wide bandgap semiconductors 

(for example CdS, ZnS, ZnSe) is that while n-type materials can be 

obtained, it is difficult to obtain significantly doped p-type 

material. It appears, therefore, that progress in developing 

shorter wavelength lasers using such materials may require the 

introduction of minority carriers without benefit of p-n junctions. 

(It must be pointed out that more recent developments suggest 

that this situation may be changing; see for example the section 

I 

I 

13 
dealing with ZnSe below). The importance of these non-junction 

methods comes about because, in the words of H. Henisch13, 

Itany nonequilibrium situation that involves an excess of carriers 

can support luminescencett. As will be discussed later on, there 

is evidence that such non-junction injection methods have already 

been used to make a diode laser (although an unstable and shortlived one). 

The present section provides a brief reveiw of p-n junctions, 

followed by a discussion of 

be used to introduce excess carriers into semiconductors. 

a number of other methods which may 

A. P-N Junction 

Carrier injection in a p-n junction can be visualized as 

follows: consider an n-type and a p-type sample of semiconductor 

material brought together at an interface. This causes electrons 

to diffuse from the n-side into the p-side, and holes to diffuse 

from the p-side into the n-side, thereby uncovering the charge 

of the fixed dopants built into the material, the space 

charge. The space charge in turn generates an electric field 

across the junction region, which causes a drift of carriers 

across the junction, in opposition to the diffusion of carriers. 

In equilibrium, the net current across the junction is zero, and 

this requirement then determines how much space charge there will 

be. to the motion 

of carriers, i.e. no carriers in addition to those required to 

establish equilibrium can flow across the interface. 

By this means one obtains a potential barrier 

When a forward bias is applied to the structure, the electrostatic potential across the junction is reduced, thus reducing 

14 
the drift current. The effect of this is that the current 

due to diffusion predominates, and extra minority carriers 

traverse the junction and show up on the other side. 

The process is controlled by the requirement for current 

continuity across the barrier, together with charge neutrality 

in some regions, and built-in fields in others. The extra 

carriers appearing across the junction are available for use, 

for example to drop down to the valence band, and in so doing emit 

light. 

Although this description is generally valid and leads to 

quantitative formulations for carrier injection, it is also 

cumbersome when a number of structures are to be compared. A much 

simpler, qualitative approach is to use band diagrams, which will 

be illustrated in the case of p-n junctions, and thereafter used 

to discuss other injection mechanisms as well. 

Figure 5(a) shows the structure of a p-n junction diode 

without applied bias, the corresponding energy band diagram, and 

the Fermi level (a line marking that energy level which has a 

probability of 0.5 of being occupied by electrons). 

requirement in drawing these bands is that the Fermi level with 

no applied bias has to be flat across the interface between the 

two materials. Figure 5(b) shows the same structure under a 

forward bias of Vf. Now the Fermi level shows a displacement 

equal to the applied voltage multiplied by the electronic charge, 

q (assuming no voltage drops across other regions of the device). 

For an intrinsic (lightly doped) semiconductor, the Fermi level 

is near the center of the bandgap, and for a well doped n-type 

semiconductor, the Fermi level moves up to or even past the edge 

The principal 

15 
of the conduction band. Thus, the position of the Fermi level 

indicates the doping level. Injection of carriers is indicated by 

the height of the energy barrier that carriers must overcome, and 

by the concentration of carriers available for climbing the 

barrier. Thus, in figure 6(a), we see the barrier for holes and 

electrons is approximately the same, and the availability of 

carriers is also about the same, as indicated by the position of 

the Fermi level relative to the band edges. Therefore, the 

amount of hole and electron injection will be about the same, as 

indicated by equal-sized arrows. In figure 6(b), showing an 

unsymmetrically-doped junction, the barrier height (from the Fermi 

level to the band edge) is different, and the availability of 

electrons is much lower, as shown by the deep position of the 

Fermi level on the n-side. This structure will have mostly 

hole injection. 

B. Schottky Barrier 

This is the interface between a metal and a semiconductor. 

The semiconductor is described as in Section I above, but the 

description of a metal requires some new terms14. Referring to 

figure 7, we first note the energy difference called the work 

function, which is the energy required to take an electron from 

the Fermi level clear out of the metal (this is taken to be the 

vacuum level). The work function for a semiconductor (or any 

other material) will in general have a distinct value. Under 

equilibrium, the band structure will appear as shown in figure 

7(a), where we see that the Fermi level is flat. The semiconductor electron affinity is the energy difference between the 

16 
vacuum level and the conduction band edge. The vacuum level in 

equilibrium must always be drawn without interruptions, and it 

must be parallel to the band edges. The barrier between the 

metal and the semiconductor is given by the difference between the 

metal work function and the semiconductor electron affinity, and 

should thus increase with the metal work function. It should 

therefore be possible to obtain a wide range of barrier heights by 

using metals with different work functions. In practice, the 

metal work function only exerts a minor effect on the barrier 

height because of states on the semiconductor surface, so that the 

barrier is largely determined by the semiconductor alone. 

Figure 7(b) shows the Schottky barrier under forward bias. 

It is apparent that electrons will be injected into the metal, 

resulting in no increase in the net carrier concentration in the 

semiconductor. At large forward bias there can be significant 

minority carrier injection, but it is not possible to obtain this 

while maintaining a high donor concentration in the semiconductor. 

C. Metal-Insulator-Semiconductor (MIS) and Metal Oxide 

Semiconductor (MOS) Devices 

These devices are related to Schottky barriers, the 

difference being the presence of an intermediate oxide or 

insulating layer. In general, there is again no injection of 

carriers in such a device, its utility coming as a method of 

controlling the electric field. However, a special version of 

this structure is discussed next. 

D. Langmuir-Blodgett or Tunneling-MIS Devices 

If the insulating layer in an MIS device is made thin enough, 

17 
there exists the possibility of minority carrier injection, i. e. 

injection of holes from the metal into the semiconductor. One 

method of depositing such films is based on the work of Langmuir 

and Blodgett with thin organic films, and some of the recently 

described light emitting devices of this type are called LangmuirBlodgett diodes15. An analysis of the mechanism has been 

presented by Card and - Rhoderick16. Figure 8(a) shows a 

relatively large step between the metal Fermi level Ef and the 

valence band. Under a forward bias the metal Fermi level remains 

pinned at the surface. When an insulator is applied between the 

metal and the semiconductor the situation changes as shown in 

figure 8(b). Now part of the applied bias develops across the 

insulator, the metal Fermi level can move, and the energy step 

between the metal and the valence band is reduced, thus favoring 

the injection of holes into the semiconductor. Ordinarily this 

would not lead to any current flow, but tunneling through very 

thin insulators, on the order of 10-50 nm, is possible and 

produces significant hole injection. It can be seen that the two 

mechanisms, barrier lowering and tunneling, require a thick 

insulator for the first case, and a thin one 

Injection efficiencies as high as 0.2 have been 

electroluminescent devices, although the current 

below 1 A/cm2. 

for the second. 

obtained in such 

density was well 

In order to construct a laser based on the tunneling-MIS 

process it will be necessary to a) greatly increase the current 

density, or b) use a very efficient material not requiring high 

current densities, such as a 11-VI compound, or c) use a radical11 

different geometry to increase the injected carrier density. 

18 
E. Heterojunction Barrier 

This structure refers to the junction of two materials 

having different bandgaps. A typical device might be the AlGaAs/ 

GaAs interface shown in igure 9(a). The spike delta E, and the 

discontinuity delta E, arise because of the differences in the 

bandgap on the two sides of the interface. Both of these can be 

minimized by some grading of the junction, and indeed, are 

not very prominent in the case of AlGaAs/GaAs and InGaAsP/InP. 

Nevertheless, they are thought to be the reason for the failure 

in many other heterostructure devices. Figure 9(b), which indicates the same structure under forward bias, shows that very 

strong electron injection into p-GaAs can occur, with at the same 

time, reduced hole injection into the n-type AlGaAs. This 

effect is taken advantage of in double heterostructure lasers to 

build up the carrier density in the active layer. Heterojunctions 

can also occur between two quite dissimilar materials. This 

subject will be taken up separately in Section V. 

F. Electron Beam Injection 

When energetic electrons impinge on a semiconductor material, 

they enter the lattice, contributing to the majority carriers 

present, but in addition they also ionize a large number of atoms 

and create electron-hole pairs above the equilibrium concentrations. The recombination of these pairs leads to light emission 

which is usually very similar to that produced in the same 

material by minority carrier injection across a p-n junction 

(differences can arise due to the additional excitation of high 

energy photons arising from other recombination paths). 

19 E 
Typical electron energies are between 20 and 40 keV. The 

average energy required for generating a hole electron pair is 

about three times the bandgap energy, or 8 to 10 eV in the 

present case. This also means that most of the energy put into 

the material is wasted as heat and only about 30 percent is useful 

in generating carriers. Finally, it should be noted that typical 

electron densities are on the order of a few A/cm2, 

and typical threshold carrier concentrations generated in the 

material by such a beam are similar to those of conventional 

p-n junction lasers, i.e. on the order of 1018-1019 It is 

believed that every p-n junction laser can be excited by using an 

electron beam, and an electron beam can also excite a number of 

materials not amenable to p-n junction fabrication. 

beam current 

~m-~. 

G. AC Electroluminescence 

This mechanism is the one operating in phosphors used in 

various flat panel displays. Since these phosphors consist 

of materials like ZnS, the mechanism of their excitation clearly 

touches on the issues considered important for diode lasers. In 

discussing these devices one has to consider the process of 

ffformingff which usually precedes their use, and which plays a 

profound role in the mechanism. According to the discussion by 

Dean17, a typical ZnS EL cell consists of ZnS particles coated 

with CuxS, sandwiched between two electrodes, one of which is 

optically transparent. During forming, a high resistance region 

is formed within this cell, near one of the electrodes, due to 

the migration of copper within the cell. The high electric field 

in this region, caused by the applied voltage, results in 

20 
electrical (avalanche) breakdown within the ZnS. The luminescent 

centers associated with the dopant (activator) such as Mn or Cu or 

C1, then capture these carriers which emit radiation upon release. 

It is to be noted that for electron-beam pumped lasing in 

similar materials, e.g. ZnS, very lightly doped or pure crystals 

are used. Thus, the excitation process involves the lattice and 

band to band recombination. In AC electroluminescence, an activator is required, just as for LED's made from indirect materials, 

like red-emitting Gap. In direct materials suitable for lasers, 

no activator is required, since the mechanism is band-to-band 

recombination. 

H. Carrier Accumulation 

This process is the increase of minority carriers above the 

equilibrium value, which occurs when a drift field accelerates 

minority carriers toward an electrode or other barrier, which 

prevents them from escaping18. It seems that local carrier 

excesses can be obtained this way, which should yield electroluminescence in suitable materials, even without p-n junctions19. 

However, the process is inefficient compared to a p-n junction, 

because of the power consumed in creating the drift current. 

I. Spark Gap Excitation 

This process is based on developing an RF or DC spark across 

an electrode held a small distance away from the semiconductor, 

both being placed in a vacuum or some other insulator. The excitation is due to electrons generated in the spark discharge which 

pump the semiconductor2'. This method, therefore, while similar 

to electron beam pumping, removes the need for a vacuum tube, a 

21 
thin window, and a focusing element, but it leaves open the 

question of laser life due to the deleterious effects of the bombardment. The mechanism of excitation may be more complicated 

than stated above, since the emission differs in a fundamental way 

from that obtained in similar materials by E-beam excitation. 

Thus, in some cases2', the material is only excited in thin 

filaments along certain preferred orientations, a process 

discussed in a number of Soviet publications under the name of 

 streamer^^ lasers22. 

J. Avalanche Multiplication (Impact Ionization) 

In this process14, available carriers are accelerated by a 

high electric field, and then cause impact ionization of the 

material. The resultant hole-electron pairs are also accelerated 

and cause impact ionization and hole-electron pair creation, the 

process rapidly building up and resulting in a high carrier 

concentration available for radiative recombination. Even though 

the efficiency of the process is generally low, it is the origin 

of light emission in many intractable materials, for example Mndoped ZnSe, NaI, and KII3. 

K. Berglund Effect 

This involves the application of an alternating voltage 

across an electrode separated from the semiconductor by an 

insulating film, essentially a MIS structure. Carriers are 

generated by avalanche multiplication during one half cycle, and 

allowed to diffuse into the semiconductor and recombine during the 

other half cycle23. It is therefore an AC method, but it is 

capable of stable emission, as demonstrated for Gap. A study of 

22 
the power efficiency of this process shows that it has between 

1/5 and 1/10 of the efficiency of a p-n junction''. 

In conclusion, it can be stated that p-n junctions remain 

the most efficient devices for carrier generation. However, other 

methods, for example avalanche multiplication, may eventually 

provide an alternative approach, provided that sample heating and 

thermal runaway can be controlled. 

SECTION IV. 

LASER MATERIALS 

A. Modifications of Existing Materials 

The most direct approach which can be taken toward obtaining 

short wavelength lasers is to look for relatively simple modifications which could be applied to currently used laser 

materials. The advantage of this method is that one hopes to 

have many of the requirements already satisfied by the ttstandardtl 

structure, so that only one new property needs to be obtained. 

Before proceeding it might be useful to restate, in a 

slightly more precise way, the basic requirements one wishes to 

have satisfied by any new laser material: 

1) The material should be an ambipolar semiconductor. 

2) The bandgap must be at least 2.5 eV at room temperature, 

and should not be greater than 3.0 eV. 

3) The bandgap should be direct. 

4) There should exist a ttcompatibletl material, to be used 

for carrier and optical confinement, which has approximately 

the same crystal structure and lattice constant, but a higher 

23 
bandgap and a lower refractive index. 

5) There should be available a bulk crystal material or an 

epitaxial layer which can be used as a substrate, and which 

has a lattice constant and crystal structure similar to that 

of the active layer. 

6) It should be possible to dope the material (both p- and ntype), to levels exceeding 10l8 ~m'~, with a mobility 

100 cm2/volt sec. 

7) The material should have adequate thermal conductivity, 

on the order of that for GaAs (0.5 W/cm deg C). 

above 

With these requirements in mind, one may be looking for 

a) materials obtained through an increase of the bandgap in 

existing laser materials, b) materials generated through the 

modification of an existing large bandgap semiconductor so as to 

yield both n- and p-type conductivity, and c) the modification of 

an existing indirect material with p-n junction capability and 

efficient radiative recombination, so as to have a direct bandgap. 

a) Bandgap Increase 

It is known that the replacement of an element of a compound 

semiconductor by an element of lower atomic number in the same 

column of the periodic table causes the bandgap to increase . 

(A well known example is the partial replacement of Ga with A1 in 

GaAs, or of As by P in AlGaAs.) 

24 

In considering such replacements, it is reasonable to start 

with a well-behaved material, such as AlGaAs, and to consider 

additions of the lightest elements from columns I11 and V, Boron 

or Nitrogen. The difficulty here is metallurgical, compounds of 

24 
these elements proving intractable by conventional growth methods. 

GaN itself is not under good control at present; however, A1N and 

GaN have been grown as films25, and films of A1GaN26 have also 

been made, and these show strong cathodoluminescence and a direct 

bandgap. However, the value of the bandgap is much too large, 

being above 4.4 eV, which would make efficient injection less 

likely. A theoretical study of AlGaAsN has been presented27, the 

calculations showing that the bandgap is direct over a broad range 

of values, depending on the composition. In general, these 

results suggest that modest lattice incorporations of nitrogen 

into well known laser materials are possible, although it may 

require new approaches in crystal growth 

I 

to achieve this. 

The growth of boron containing compounds appears somewhat 

more difficult24. Boron phosphide is a translucent crystal of 

the sphalerite structure, which is also the structure assumed by 

boron arsenide. Thus, the prospects for bandgap modification 

using’B are somewhat more distant than those using N. 

Additions of N and B may also be considered in GaAsSb and its 

alloys, but this seems less promising because of the smaller 

bandgap of the starting material. 

To summarize, the incorporation of nitrogen into existing, 

well studied materials, such as InGaAsP or AlGaAs, seems feasible, 

although the metallurgical problems may not be trivial. 

b) Attainment of p-n Junctions in Wide Gap Materials 

The main interest here is in the modification of 11-VI 

compounds which have all the desired attributes except that they 

cannot be made p-type. Good examples are ZnO, CdS, and ZnSe, all 

25 
of which have yielded excellent lasers under E-beam pumping, i.e. 

using a non-junction method for generating carriers. 

It is a universal observation that the difficulty of 

obtaining ambipolar conduction (conduction by electrons and holes) 

increases with increasing bandgap. As mentioned earlier, this is 

generally felt to be due to the tendency for self-compensation, 

which is driven by the large energy gain from compensation and 

the small energy required for native defect creation. 

Another possible difficulty arises because the usual 

acceptors are much too deep in the bandgap to be ionized at room 

temperature12. However, Li is a shallow acceptor in CdS28; it 

has also been used to obtain p-type doping and blue light 

emitting p-n junctions in ZnSe2’. Recently, using metalorganic VPE, 

p-type carrier concentrations of 10~~40~~ cm-3 as well as 

light emitting diodes have been achieved3’. However, because of 

the tendency of Li to drift rapidly through most semiconductors, it 

is not yet clear whether this is a useful approach. 

Since the tendency for self-compensation in wide bandgap 

materials is an equilibrium process, it may be possible to 

overcome it by using a strongly nonequilibrium growth process, 

such as MBE. Thus, new attacks on the growth of 11-VI materials 

may prove fruitful. 

In summary, there is a good possibility that many wide gap 

semiconductors, particularly those of the 11-VI classes, may be 

successfully grown to have ambipolar conduction. However, it 

appears use a material with a bandgap unnecessarily 

large. The interesting candidates, from the present point of 

view, are materials showing efficient lasing under cathodowise not to 

26 
luminescence. 

c) Conversion to Direct Bandgap 

Although the modification of the band structure to produce 

a direct bandgap may appear difficult, alteration of the bandstructure is not unknown, for example through the application of 

hydrostatic pressure. According to a recent analysis31, it was 

shown that a quantum-well superlattice composed of layers of an 

indirect semiconductor separated by suitable barrier layers should 

have a direct bandgap. The change in the bandstructure is I 

attributed to the presence of the new periodicity of the 

superlattice. While there has been no experimental confirmation 

of this idea, it is true that AlGaAs/GaAs superlattice structures 

do show a reduction of the threshold for short wavelength devices, 

as expected if the direct/indirect crossover had been shifted to 

higher energies. On the other hand, the threshold reduction has 

also been explained on the basis of a more conventional analysis. 

What is wanted is a convincing demonstration of the concept, using 

an indirect material available in both conductivities, and known 

to be capable of efficient radiative recombination (such as Gap/- 

AlGaP or just AlGaP, where the two components of the superlattice 

could be obtained through changing the ratio of Ga to Al). 

I 

Another proposal, also based on a theoretical study, is to 

introduce small interstitial atoms into the sphalerite type 

lattice of a typical indirect semicondu~tor~~. Itwas calculated 

that Gap, for example, would become a direct semiconductor through 

the insertion of helium atoms, although this structure was not 

I 

considered to be stable. However, stable compounds of the I-11-V ~ 

27 
type, such as LiZnP and LiZnN were also predicted to have a direct 

energy gap3 3. 

In summary, two possibilities have been mentioned for 

changing an indirect to a direct bandgap, and thereby achieving 

lasing transitions in suitable materials. Up to now, these 

possibilities have not been verified experimentally. 

B. Prospects for the Synthesis of New Materials 

It is certainly possible that new materials may be invented 

which will yield p-n junction type lasers at shorter wavelengths. 

The enormous success of compound semiconductors has shown that 

no penalty need be paid in using a multi-component material, 

compared to elementary or even binary compounds. Thus, InGaAsP 

lasers have been brought to a very high degree of perfection, 

in spite of early predictions of difficulties associated with 

non-radiative recombination. The two aspects of new materials 

to be discussed are the likelihood of there existing untried 

combinations of elements showing semiconductor behavior, and the 

likelihood of such a new material having a direct bandgap. 

The question of how many semiconductors there are is one of 

considerable importance, which has been explored by several 

authors. Goryunova's rules24 are based on the need to have a 

tetrahedral atomic arrangement in the lattice as this shows the 

covalent bond, characteristic of semiconductors. For the tetrahedral arrangement in diamond-like structures, the first rule is 

that the average number of valence electrons per atom must equal 

four. The second rule is that the binary analogues of group IV 

semiconductors must have a llnormal valence'', i.e. that the number 

28 
of electrons given up by the cation must equal the number of 

electrons needed to achieve a total of eight on the anion. These 

rules lead to three classes of binary compounds, namely III-Vrs, 

11-VI'S, and I-VII's. If we assume that there are four elements 

in each column of the periodic table which participate in 

Ivinterestingvv compound formation, we obtain a total of 48 possible 

compounds in the binary series. Table 3 lists the total number of 

possible compounds for some higher multinary combinations, based 

on Goryunova's deduction of acceptable classes: 

Table 3: Number of Possible Semiconductor Compounds. 

Material type Number of possible compounds .................................................... 

binary 

ternary 

quaternary 

quinary 

48 

320 

9472 

43,008 

It can be seen that the number of possibilities, without 

going beyond quinaries, is already very large. The ternary 

classes are: 

For the list of quaternaries and quinaries, it is best to 

consult the original reference24. 

However, the above list is not yet complete, because there are 

also @Idefectvv crystal structures which include vacancies in 

certain lattice positions. For these substances, the number of 

29 
valence electrons per atom need not equal four. Thus, a more 

general rule24 is that the number of electrons per atom in binary 

compounds with possible semiconducting properties must be between 

2 and 7, and this now includes both filled and defect type 

structures. For binaries, the expanded list of classes now 

numbers 23 instead of the previous 3, and the total number of 

substances will be a great deal larger than estimated above. Past 

experience also indicates that a large fraction of these compounds 

will have their bandgaps in the visible range, as long as elements 

near the first or second rows of the periodic table are used. 

A more recent approach to similar problems, with slightly 

different rules is given by M~oser~~. Inaddition to the 

compounds crystallizing in the diamond, wurtzite or sphalerite 

structures discussed so far, he also considers transition element 

compounds crystallizing in the pyrite, marcasite and arsenopyrite 

structures, which may also show semiconducting behavior. 

It is seen that the number of possible semiconducting 

compounds, when allowance is made for multinary structures and for 

classes of materials not previously thought of as semiconducting, 

becomes too large to be enumerated. 

Predicting whether a hypothetical multinary compound has a 

direct bandgap requires extensive work. Van Ve~hten~~ has presented calculations of direct and indirect bandgaps of a number 

of binary, tetrahedrally coordinated compounds of the diamond, 

sphalerite and wurtzite structures. This approach, based on 

electronegativity differences, provides a very good estimate of 

the bandstructure of such compounds, and has been used to derive 

the crystal structure and bandgap of A~G~AsN~~. 

30 
For existing materials, this determination is usually made 

experimentally, through the study of absorption or electroreflectance spectra. Occasionally, if all available members of a 

certain class of compounds have a direct bandgap, one may expect 

that as yet unsynthesised members will have a similar band 

structure. In general, however, it requires detailed calculations to predict the bandstructure of new multinary compounds. It 

is likely, however, based on the experience obtained with existing 

materials, that a large number of new materials will have a 

direct bandstructure. Thus, the prospects for synthesizing new 

materials with desireable properties are very good, but it is 

difficult to make predictions concerning their composition. 

The next section takes a different approach, necessitated by 

the difficulties discussed above, in providing a tabulation of 

materials culled from the literature. Among these are compounds 

which have shown promise as light emitters or lasers without 

having reached practical levels, and in general materials which 

might, after further development especially by using improved 

growth processes, become useful as short wavelength lasers. 

C. A Table of Materials 

Various compounds of interest in developing new laser 

materials have been described in the literature. The table below 

lists some of these, and for a considerable number of them also 

supplies the bandgap in eV, an indication of whether they are 

direct (D) or indirect (I), the crystal structure, and comments. 

31 
~ 

Material 

Class 

------.-- 

CdP2 

ZnP2 

ZnSe 

CdS 

CdSe 

BeTe 

ZnS 

ZnO 

ZnTe 

GaN 

InN 

BN 

BP 

Gas 

Gas2 

GaSe 

Bandgap 

----.---- 

2.1 

2.14 

2.7 

2.4 

1.77 

2.9 

3.8 

3.25 

2.3 

3.5 

1.9 

6.4 

3 (2) 

2.5 

3.5 

2.1 

BINARIES 

D/I# 

structure 

---------.--- 

11-v, 

I, chalcop. 

I, chalcop. 

11-VI 

D, 

sphal./wurtz. 

D, 

sphal./wurtz. 

D, wurtz. 

D, sphal. 

D, 

sphal./wurtz. 

D, wurtz. 

D, sphal. 

D, 

wurtz. 

D, wurtz. 

I, cubic 

111-v 

p-n junct. ,LED, 

E beam laser 

E beam laser, 

diode? laser 

0pt.pump laser 

unstable 

E beam laser, 

LED 

E beam laser 

E beam laser 

n-only;Be,Li,Dy are 

promising p-dopants 

pn junct.,LED in UV 

I, sphal. 

111-VI 

I, layered injection EL @77K 

I 

D, layered 

36 

37 , 37a 

29,30 

39,40,41 

41 

37b 

37c, 37d 

118 

37e 

38,42 

43 

44 

24 

45,46 

37 

46,47 

32 
A12S3 

A12Se3 

Ga2S3 

A12Te3 

Ga2Se3 

In2'3 

Ga203 

D, wurtz. 

wurtz. 

SnOZ 

sharp CL in U.V. 

n only 

~ 

4.1 

3.1 

2.5 

2.2 

1.9 

2.75 

2.5 

3.6 

III,-V13 6 

wurtz. 

wurtz. 

wurtz. 

wurtz. 

sphal. 

D, corundum? 

I, corundum 

IV-VI* 

D, rutile I 

48 

48 

24 , 48 

48 

48 

37 

37f 

37 , 37g 

Znl-x Cd x" C 

CdMgTe 

CdSSe 

2.9-3.0 

2.3-2.7 

2.6 

2.95 

1-2.5 

2.1 

>=3.5 

< 3.5 

(2.1) 

TERNARIES 

D, sphal. 

D, sphal. 

D, sphal. 

D , sphal. 

D, wurtz. 

Comments 

11-VI 

E beam las.@430 nn 

E beam laser 

ambipolar, weak E1 

E beam laser 

ambipolar,LED 

opt.pump laser 

I11 7 

49,50 

51 

51 

50,51,52 

53 

41 

26 

54 , 55 

33 

33 
CuAlS2 

CuAlSe2 

CuGaS2 

AgGaS2 

Ag2HgI4 

ZnGa2S4 

CdGa,S4 

CdGa2Se4 

CdIn2S4 

ZnIn2S4 

HgGa2S4 

ZnGa2Se4 

ZnSiP2 

CdSnP2 

CdGeP2 

CdSiP, 

ZnGeP2 

MgSiP2 

MgSiN2 : Ec 

3.5 

2.7 

2.5 

2.7 

3.18 

3.58 

2.57 

2.3 

2.86 

2.79 

2.17 

2 

1.17 

2-5 

2.21 

2.25 

2.24 

~~ 

1-1 I I-VI2 

D, chalcop. 

D, chalcop. 

D, chalcop. 

D, chalcop. 

ambipolar,red PL 

ambipolar 

orange EL,pn jun., 

las. 500 nm by 

opt. pump 

n-only,EL,CL,las. 

462 nm by opt.pumF 

I2-II-VII, 

I I -I I I2 -VI , 

I yellow color 

D,stannite/ 

chalcop. 

D, chalcop. 

D, chalcop. 

I, spinel 

D, PolYtYPes 

D, chalcop. 

D,stannite/ 

chalcop. 

II-IV-v* 

1,chalcop. 

D, chalcop. 

D, chalcop. 

(D) , chalcop. 

(D) , chalcop. 

D, chalcop. 

34 

ambip.,EL, 

las.by E-beam 

EL 

n, EL. 

layered, ambip. 

n-only; las. by 

opt. pump 

green EL 

~ 

56 

56 

57 , 58,59 

58,60,61 

48 

62 

48,62,63 

48,62,64 

65 

48,62 

48,62 

62 

66,67 , 68 

69 , 70 

66 

71,72 

71,73 

74 

61 
CdMnSe 

I, layered 

1.95 

Red PL 

D 

D, layered 

D, layered 

II-VI-VI1 

PL 

opt.pump.las.638nm 

magnet. tunable 

D, layered 

III-v-VI 

0.3 < X < 1 

green PL 

75 

76 

Material 

Class 

-.------- 

AlGaAsN 

CdInGaS4 

Cd (InGa) 

ZnCdIn2 S4 

Cd3InGaSg 

Zn31nGaS6 

ZnSe-GaP 

Bandgap 

0 - - - - - - - - 

>2 

2.7 

2.6-3.2 

2.76-2.86 

2.43 

3.26 

2.24-2.7 

QUATERNARIEB 

III-v 

I theory Only 

D, sphal. 

D 

III-v-II-VI 

D, sphal. I ambipolar 

Ref. no. 

27 

77 

78 

79 

77 

80 

81,82 

35 
SECTION V. 

HETEROSTRUCTURES 

The interest in using heterostructures is that by this means 

one may construct a device using a p-type semiconductor of one 

material and an n-type of another, and thus hope to obtain 

minority carrier injection without having both p- and n-type 

doping in the same crystal. Thus, if one could deposit a wide 

bandgap p-type semiconductor onto a crystal of n-type ZnO, hole 

injection into ZnO might be obtained. 

A difficulty with heterostructures is that a change in 

composition at the heterointerface usually creates a defect layer 

which causes nonradiative recombination, and thus reduced efficiency for the device3. 

recombination velocity, S, which then plays a role in determining 

the laser threshold. If S is large enough, the threshold will be 

increased, and if it is below lo3 cm/sec, its contribution to the 

threshold will be negligible3. The usual view of interface 

recombination is that it is reduced by selecting both materials 

to have the same lattice constant and crystal structure, as is the 

case with GaAs and various combinations of AlGaAs. According to 

this model, the thickness of the active layer also contributes to 

the effect, a thinner layer leading to increased surface recombination. It has been shown83 that the recombination velocity at 

a presumably typical AlGaAs/GaAs interface is 450 cm/sec, in 

other words well below the critical value given above. However, 

as first pointed out by Ettenberg and Kre~sel~~, for active layers 

with thicknesses in the 50 to 100 nm range the low threshold 

current densities actually measured indicate that the recombiThis effect is characterized by a surface 

36 
nation velocity has to be less than one would expect. 

Recent studies85 of a similar effect in quantum Well lasers 

attribute the decrease of the interface recombination velocity to 

quantum phenomena occurring in thin layers, particularly the 

peaking of the electronic wave function at the center of the layer 

(well), away from the interfaces. Quantum well lasers may 

therefore show much more tolerance for faults at heterointerfaces 

than the lasers studied so far. 

Another feature of heterostructures is the presence of an 

energy spike caused by the difference in bandgaps and work 

functions between the two sides of the structure86. As shown in 

figure 9, this shows up mostly in the conduction band (for AlGaAs/ 

GaAs), and is usually assumed to occur in the conduction band in 

other systems as well. As mentioned earlier, grading the 

transition region reduces the magnitude of these energy discontinuities. Nevertheless, their presence can impede the injection 

process, which is why one prefers heterojunctions with only a 

modest difference in bandgap energies. 

It is clear that in regard to structural perfection, the 

AlGaAs/GaAs and the InGaAsP/InP interfaces satisfy the requirements imposed by laser operation. So far, no pairs of larger 

bandgap materials seem to have been found which show similar 

perfection. However, it appears that the losses at heteroboundaries in many novel systems have not been studied 

sufficiently to know if they can be reduced by special growth 

techniques. 

The following table shows some heteropairs in the short 

wavelength regime which have been discussed in the literature: 

37 
Table 4: Short Wavelength 

Active layer 

AlGaInP 

ZnSTe 

ZnTe 

CdS 

cladding layer 

AlInP 

ZnSe or ZnSSeTe 

ZnSe 

CuGaSa 

Heteropairs. 

emitted 

wavelength 

550 nm 

500 nm 

540 nm 

510 nm 

Ref. 

87 

87 

88 

89 

None of ,hese structures have so far produced 

emission, nor do they approach the AlGaAs/GaAs 

stimulated 

system in 

efficiency. Thus, much work remains to be done in this area. 

SECTION VI. 

FREQUENCY MULTIPLICATION 

Changing the output frequency of a semiconductor laser is 

another approach to obtaining short wavelength emission. 

this converting the diode laser output directly, 

most presently used devices go through an intermediate stage, in 

which the diode laser is used to pump a crystal laser, such as 

Neodymium YAG. (The reason for this is that the direct output of 

the diode laser does not posses the modal purity required for 

high conversion efficiency. Thus, diode lasers can mode jump, 

i.e. switch the wavelength; they may have a complicated lobe 

structure in the output beam which may change during operation, 

and their power output as well as wavelength depend on the 

temperature) . 

Although 

can be done by 

The diode-pumped YAG output is passed through a non-linear 

38 
optical material which generates the second harmonic frequency, 

with phase matching used for both the fundamental and the harmonic 

frequencies. One popular nonlinear optical material is Mg0:- 

LiNb03, which can be placed inside the cavity. The YAG 

emissiong0) at 1.047 um is converted to 523.6 nm, but with two 

conversions; the overall efficiency is low. A schematic diagram 

of the arrangement is shown in figure 10. 

Another recent result, using an external cavity for the 

doubler, is an output of about 30 mW at 532 nm from an input of 

52.6 mW from a diode laser pumped Nd:YAG laserg1. The overall 

power efficiency was on the order of a few percent. The highest 

output power yet reported is the achievement of 180 mW at 532 nm 

from a Nd:YAG laser pumped with several laser diode arrays 

combined via a fiber optic bundleg2. The overall efficiency was 

4.4 percent. Blue emission by frequency doubling has also been 

obtained. Thus, Matsushita Co. has reported direct conversion of 

60 mW of diode laser emission to an output of 0.2 mW at 420 nmg3. 

Frequency doubling of a diode laser pumped Nd:YAG to 473 nm using 

KNb03 has yielded 3.1 mWg4. 

Another scheme is to use frequency summing in a nonlinear 

crystal, the output from two lasers producing the sum frequency. 

Output in the blue, although.at a small power level, has actually 

been demonstratedg5. This may have some advantage if the phase 

matching condition requires a particular frequency. Related to 

this scheme is parametric mixing in a suitable crystal, but this 

does not offer any obvious advantages. 

A different approach is based on two photon pumping (also 

called upconversion), where for example, one photon pumps near 

39 
792 nm and the other near 840 nm, with laser output at 550 nmg6. 

The levels are provided by Er3+ in YA103. As both pump wavelengths are reachable with current diode lasers, this scheme may 

yet turn out to be useful. Two photon pumping has also been used 

to generate 380 nm lasingg7. Pumping of a suitable phosphor (for 

example LaF3:Yb, Er) with multiple photons near 930 nm has been 

tested a number of years ago as a way of obtaining visible 

LED's 98,99 

SECTION VII. 

ELECTRON BEAM PUMPED LASERS 

All of the usual semiconductor laser materials can be excited 

with electron beams, and several II-VI materials, particularly 

CdS, CdSe, ZnSe, and ZnO, have been used to construct E-beam 

lasers. Because of heat dissipation problems, pumping is typically by 50 ns pulses. 

The main objections to this method of laser pumping is the 

cumbersome high voltage supply, the need for a vacuum tube device 

to generate the electron beam, the limited life of the window for 

this beam, and questions about life in the semiconductor exposed 

to electron bombardment. The important question is whether these 

lasers are practical devices, i.e. do they meet the criterion of 

small size, easy modulation, and life? 

Addressing some of these concerns, workers at Phillips 

Laboratories loo have shown very small, sealed tube designs 

operating at room temperature. 

obtained from electron beam pumped lasers (100 kW peak)"', and 

Very large power outputs have been 

40 
the literature abounds in descriptions of devices. 

Papers dealing with degradation processes in these lasers 

are more scarce. 

to the charging of various impurity centers, and an irreversible 

process due to the generation (in the case of CdS) of sulfur 

vacancies lo2. 

ation dose, which can introduce damage (for 50 keV electrons) at 

a value as low as a few times electrons/cm2. In addition 

there is a catastrophic degradation threshold at a laser output 

power of 0.2-3 MW/cm2. These results are considered due to the 

faults in the quality of the CdS crystals used, so that there is 

hope of greatly improved life based on materials and engineering 

advances. 

There appears to be a reversible degradation due 

Thus, the life is specified in terms of the irradiSECTION VIII. 

QUANTUM WELLS AND SUPERLATTICES 

If the active layer in a conventional double heterostructure 

injection laser is made thin enough, the carrier distribution and 

energy levels in this region become subject to quantum mechanical 

laws. Such a device is called a single quantum well (SQW) 

laserlo3. The active layer is now looked on as a potential well, 

with energy levels quantized to a few allowed values selected out 

of the continuum of states in the standard bandgap model. In 

particular, since the lowest allowed level is above the minimum 

allowed without the well, the emission wavelength should be 

shorter. Some of the other advantages expected from this 

structure are a lower sensitivity to temperature, and a lower 

41 
threshold current. 

an infinite one dimensional potential well, and the first two 

levels in the more realistic finite well occurring in the semiconductor laser. 

Figure 11 shows the first two energy levels in 

The dimension being considered is the layer thickness, 

which is also the growth direction. In this direction, there is 

only limited freedom of motion for carriers because of various 

restrictions due to the quantization; the carriers are free to 

move in the other two directions. 

two-dimensional system, in contrast with true two-dimensional 

systems such as planar waveguides. A consequence of this 

restricted dimensionality is that the density of states is also 

restricted to a series of discrete values, as shown in figure 12. 

Therefore, the density of states function never reaches zero, even 

at low temperatures. Other advantages flowing from the restricted 

density of states function are a higher gain at low injection 

levels, a smaller gain dependence on temperature, a lower absorption coefficient in unexcited material at the lasing wavelength, 

and an overall lower value for the density of states function 

compared to the three dimensional case. All of these factors are 

such as to suggest that the threshold current in quantum well 

lasers will be reduced. However, the maximum possible gain is 

also reduced, implying that SQW lasers are more useful in high 

quality materials with few nonradiative centers. 

Weisbuch lo3 calls this a quasiThe deposition of a series of thin layers of alternating 

bandgaps results in a multi quantum well (MQW) structure, which 

has the advantage that the net density of states (being the 

product of the one-well density of states by the number of wells) 

42 
is increased. It is possible to reach much greater population 

inversions in such devices. The optical overlap between the gain 

region and the mode in SQW tends to be very small, and this also 

can be increased by going to MQW's. 

The design of quantum well lasers has been greatly advanced 

by Tsang 1 in developing separate confinement heterostructures combined with a graded index region, both together being 

called the GRIN-SCH-QW laser (graded-index separate-confinementheterostructure quantum-well). 

This structure, briefly discussed in section I1 and shown in 

figure 4, combines the advantages of a single quantum well laser 

with the improved optical overlap caused by the index grade, to 

yield the most advanced laser structure available at present. 

Together with one or another scheme for lateral confinement, it 

has been used in many current devices with record setting performance. The demands for perfection in layer growth for these 

devices are severe, MBE being the growth method usually employed. 

In the MQW's discussed so far, the layer thicknesses are 

assumed large enough to prevent interactions, but if the layers 

are thin, tunneling between them can occur. In this case, and 

assuming the layers are regular enough, there arises a new 

coherent structure called a superlattice state. 

If the alternate layers are composed of materials of 

different bandgaps, then the superlattice assumes a bandgap which 

is intermediate between these two values, the actual value 

depending on the layer thicknesses. One can see therefore, that a 

material with any desired bandgap can be constructed. This has 

been demonstrated in InGaAsP, made up from alternating layers of 

43 
106 InP and InGaAs . 

So far, superlattice devices have not shown the spectacular 

results of SQW or MQW lasers, but they are the focus of tremendous 

activity, particularly in areas where conventional approaches have 

been pushed to the limit. Stimulated emission has been obtained 

by optical pumping of superlattices composed of alternate layers 

of ZnSe and ZnMnSe . Emission was obtained in the 451- 

455 nm range at temperatures as high as 80 K. Based on a 

theoretical analysis, it has been claimed that direct optical 

transitions can be obtained in a certain superlattice geometry 

assembled from materials having only indirect transitions in their 

normal state3'. 

107,108 

Doping superlattices are structures obtained by spatial 

modulation of doping (so called n-i-p-i structures) log. 

non-radiative recombination rates are strongly suppressed, 

compared to more moderate suppression of radiative recombination. 

Thus, the possibility exists that emission in some indirect gap 

semiconductors may be enhanced over that occurring in standard 

bulk geometries. It has also been speculated that such devices 

made from indirect material may be overcompensated to yield a 

direct bandgap . 

In these, 

109 

Another new phenomenon is "resonant tunneling1t103. Referring 

to figure 13, this occurs when the Fermi level of the injecting 

material (GaAs) on the left side of a structure composed of two 

barriers and a quantum well, is at the same energy as the lowest 

allowed energy state of the well. This is achieved by applying 

the correct bias to the device, in which case there is a peak in 

the current flow through the structure, as demonstrated both 

44 
theoretically and experimentally. Resonant tunneling is the basis 

for a new approach to the injection of hot carriers, based on a 

staircase of energy levels obtained from a series of quantum 

wells with progressively changing widths. Carriers traverse each 

well by resonant tunneling and are able to enter the semiconductor 

at an elevated potential. Such a structure is called a Itvariable 

spaced superlattice 1,110. 

As mentioned elsewhere, hot electron injection is a 

satisfactory method for excitation of many 11-VI compounds, such 

as CdS, ZnO, ZnSe. Brennan and Summers propose variable spaced 

superlattice injection into Mn activated ZnSe in order to obtain 

an electroluminescent display. The distinction between this and 

the requirement for stimulated emission in a similar structure may 

be one of geometry only. The reason for this is that besides the 

impact ionization of an activator center, which is the case in the 

above scheme, hot electrons may also ionize the crystal, and in so 

doing generate hole-electron pairs. These are soon thermalized 

and thus recombine across the bandgap in the usual way, producing 

near band edge luminescence or lasing. 

Strained layer quantum wells or strained layer 

superlattices are obtained when there is a lattice mismatch 

between the layers which is not allowed to be accommodated through 

dislocations. This is accomplished by keeping the layers thin 

enough and thereby limiting the amount of strain energy built up. 

The lattice misfit is on the order of a few percent, and the 

layer thickness is typically less than 5 nm. 

Besides further increasing the range of available materials 

combinations, new effects are expected to arise through the 

45 
tetragonal strains now occurring in the structure; for example 

the change of certain indirect gap transitions into direct 

transitions112, a decrease or increase of the bandgap113, and the 

blocking of misfit dislocation motion by these strained layer 

superlattices. Strained layer quantum well lasers in InGaAs/GaAs 

have been made and their properties studied It appears that 

they are well behaved, with promising early life test results. 

114 . 

SECTION IX. 

SPECIAL TOPICS 

This section deals with certain materials which have been 

studied for some time, by many different groups, with the object 

of developing electroluminescent devices or lasers. Their 

inclusion here is in the nature of a status report. 

A. Sic 

A well known difficulty with this material is the great 

number of crystal structures which occur. The 6H polytype 

configuration has been much studied as an electroluminescent 

material because of its large bandgap and the ability to dope it 

both p- and n-type. It also has notable drawbacks, namely an 

indirect band structure, a tendency toward polymorphism, and 

great difficulties in growth. 

This situation has been improved in recent years by growing 

it from the vapor or by LPE onto substrates of Sic obtained by 

other methods115. Using A1 for the acceptor and N for the donor, 

a quantum efficiency of 0.01 percent has been achieved. Cathodo46 
luminescence studies of p-n junction type diodes show spectra 

similar to those found in EL, with no evidence of stimulated 

emission. Injection electroluminescence has also been obtained 

from amorphous but not at wavelengths as short as the blue. 

None of the polytypes of Sic have a direct bandgap117, and 

the main interest in this material is therefore as a component in 

multinary compounds. 

B. ZnO 

This material is a semiconductor with a direct bandgap large 

enough (3.2 eV) to satisfy the requirements for blue emission. 

Unfortunately, it has not so far been possible to grow the 

material with p-type doping. 

However, ZnO shows intense laser emission when excited by an 

electron bearn118. In diode form, experiments have been reported 

for N-I-Metal diodes, where the N layer is ZnO and the I layer is 

obtained by oxidation of the crystal surfacell'. The emission has 

a peak near 5100 A, and the external quantum efficiency is estimated to be on the order of 0.001 percent. The current-voltage 

characteristic is quadratic rather than the exponential characteristic of minority carrier injection. 

A study has also been made of a Schottky (i.e. metal-semiconductor) diode1l0. In this case the spectrum was much broader, and 

the efficiency was similar to the above value. 

It seems reasonable to conclude that it is the lack of an 

efficient injection system which has hindered progress using ZnO. 

This material therefore becomes a prime candidate for experiments 

with novel injecting methods. It is also possible that p-type 

47 
material may be achieved through methods similar to those 

currently applied to ZnSe (new dopants, e.g. lithium, ion implantation, etc.) . 

I C. ZnS , 

, In this material it has also proven impossible to make welldoped p-type layers, thus precluding p-n junction devices. Blue 

, emission was obtained from MIS type structures with a quantum 

efficiency of 0.05 percent . Lasing was obtained at 345 nm 

using pulsed electron beam excitation . Lawther and Woods 

prepared both Schottky type 

the latter being generated by E-beam evaporation of ZnS. Electroluminescence at 445 nm was attributed to injection of holes from 

the metal into the semiconductor. The effect of different thick121 

122 

and MIS type 123 diodes, the I region in 

nesses of the I layer was also investigated. Efficiency was 

limited by shortage of minority carriers, and the devices were 

characterized by relatively high series resistance. 

In the above study 123 and in a later one 124 , it was shown 

I that the electroluminescence is much weaker, or vanishes if there 

is no interfacial insulating layer, and that the peak brightness 

occurs for relatively thick I layers (500 A or more). These 

findings suggest that the operating mechanism is much like the one 

I discussed in the section on Langmuir-Blodgett diodes. 

D. ZnSe 

This is probably the most interesting compound semiconductor 

in the 11-VI system. Its bandgap is 2.67 at room temperature, I 

and it is direct, so that recombination is obtained through the 

efficient band-to-band process. It suffers from the usual problem 
that p-type materia! is difficult to make, so that 

junction type of injection has so far not been realized. 

efficient p-n , 

In a fairly recent paper 125 it is shown that growth of 

ZnSe from Se solution yields p-type crystals, and that p-n 

junctions can be made by diffusing Ge into such material. 

Resultant diode properties appear entirely consistent with this 

interpretation. Edge emission at 480 nm was obtained at room 

temperature, and the intensity increased linearly with current in 

the range studied. No output efficiency is given, but the 

brightness of 2 mcd at 2 mA suggests that the efficiency was low. 

ZnSe can be made to lase at room temperature when excited with an 

electron beam, and when photopumped126. Methods of growth of ZnSe 

I 

(and ZnS), and of heterostructures on ZnSe films have been 

reported 127 . 

It has been shown that MOCVD grown films of ZnSe can be excited 1 

into stimulated emission with an electron beam128. When such 

films were assembled into Langmuir-Blodgett diodes, band-to-band 

recombination radiation was observed12'. Short wavelength emission 

at room temperature consisted of peaks at 460 and 500 nm. This 

was achieved by growing a film of ZnSe onto a GaAs substrate 

using MOCVD, and then depositing a thin organic insulator under 

the metal electrode. The 460 nm peak is due to band-to-band I 

recombination, but the presence of the 500 nm emission suggests 

a strong influence of defects. 

Recently13', p-type ZnSe was obtained by MOCVD growth onto 

GaAs, using lithium nitride as the dopant source. Aluminum doped 

I 

ZnSe was then grown by the same method on top of the p-type 

material. The emission at room temperature peaked at 467 nm, and 

I 

49 
there is a lower intensity broad band peaking near 540 nm. In 

this experiment, substantial p-type conductivity with a carrier 

concentration up to 9 x 1017 (resistivity = 0.2 ohm-cm) has been 

obtained. In order to improve the lasing performance of ZnSe 

it is helpful to sandwich the material 

index. ZnS has been proposed for this purpose, and stimulated 

emission was obtained at room temperature by electron beam 

pumping such a structure . 

between layers of lower 

131 

A significant observation is that most ZnSe growth processes 

lead to the generation of defects, presumably vacancies. One 

exception was the growth by liquid phase epitaxy, from melts of Bi 

or Sn132. One of the newest attacks on this material is the 

growth of strained layer superlattices. The materials used 

were ZnS and ZnSe, with a lattice misfit of 5 percent 133 . 

E. GaN, InN 

GaN is a semiconductor with a bandgap of 3.5 eV, and if 

suitably doped, could serve as emitter for any visible wavelength. 

Since its band structure is direct, it was expected to be a good 

short wavelength laser material. To date, these hopes have not 

been realized because of the difficulty in obtaining p-type 

material. There have been a number of studies of EL in various 

configurations; a typical work134 uses MIS contacts, the 

insulator being A1203 or NaI, and the metal Au. 

to the n-type GaN was In, and GaN was deposited by a vapor 

phase process onto sapphire substrates. Emission in the blue 

region was observed at room temperature and at 77K, with low 

The ohmic contact 

efficiency. 

50 
Another approach is to produce a near intrinsic layer of GaN, 

135 thus making a i-n or even n-i-n structure . The i region is 

produced by doping GaN with zinc during growth. Some of the 

interesting observations concerning that structure are the 

variation of the emission wavelength between red and blue 

depending on the drive current, and the emission of polarized 

radiation. The spatial dependence of the luminescence was investigated, both by CL and by EL studies. However, no lasing 

emission was reported. 

InN is a direct bandgap semiconductor with E = 1.9 eV, and g 

alloys of InGaN allow the selection of intermediate bandgaps. Good 

results in the growth of InN have been reported using microwave 

excited MOCVD43. It is interesting to note a significant recent 

result in a similarly intractable material, Boron Nitride. W 

emitting p-n junctions in the cubic form of this material have 

been obtained by a special high pressure growth process 136 . 

Since GaN exists in both the wurtzite and the sphalerite 

(cubic) form, it may be advantageous to consider the cubic 

structure, and such material has recently been grown using 

MOCVD137. Thus, GaN and its alloys remain promising materials, 

but because of difficulties in growth which presumably are 

responsible for reduced efficiency in compensated material, there 

has been no progress so far toward achieving stimulated emission. 

F. DiamonU 

This material is of interest because it is an elemental 

semiconductor with a bandgap of 5.6 eV. Current experiments are 

conducted on naturally occurring p-type semiconducting type IIb 

51 
material138. 

EL at the many contact points between the individual silver grains 

and the diamond. The mechanism is associated with the existence 

of high (at least 105 V/cm) electric fields, and is likely an 

impact excitation process. Luminescence has been observed in ntype synthetic diamonds when a current is passed between silver 

With silver paint contacts it is possible to obtain 

electrodes. Neither this nor the above process are apparently 

efficient. 

Diamond has been used as a host for a color center laser, 

the center being due to nitrogen. Stimulated emission at 530 nm 

at room temperature has been obtained under optical pumping . 140 

The electroluminescent brightness observed in synthetic diamond 

doped with boron or arsenic behaves in a manner consistent 

with impact ionization, which can occur if there are internal 

barriers. The field intensity at these crystal irregularities is 

considered to be sufficient to excite color centers. 

Electron beam excited diamond produces luminescence similar 

to that obtained in electroluminescence experiments. In neither 

case is there any evidence of high efficiency. Another difficulty 

is the well known complexity of growth requiring very high 

pressures. 

0. CdS 

With a direct bandgap near 2.4 eV, this material has yielded 

electron beam and optically pumped lasers at 490 - 530 nm. 

Furthermore, it has been the subject of various attempts to obtain 

more directly excited diode lasers. Lasing has been obtained by 

driving a sample with a vacuum arc which supplies electrons doing 

52 
the pumping2'. This is basically a scheme for electron beam 

pumping without an electron gun and focusing lens. Stimulated 

emission was also obtained by using RF pulses of 10-15 kV applied 

to a 200 um gap between the crystal and an electrode2'. Here 

again, it is possible that electrons were generated by an air 

breakdown mechanism. However, emission was observed to occur 

along certain preferred directions in the crystal, yielding what 

is now called a streamer laser22. 

Directional breakdown is thought to be caused by the applied 

electric field, (which has to be on the order of lo6 V/cm) and 

may be initiated by microinhomogeneities in the crystal. It has 

been argued by the Soviet workers, that it might be possible to 

obtain this conduction in a streamer channel without causing 

damage to the material, by preventing excess current flow after 

breakdown. The applied voltage is in the kV range, but very high 

powers have been obtained from such devices. 

Some form of breakdown seems to be the source of pumping 

carriers in Nic01l~s~~ early attempt to obtain diode lasing in 

CdS. Two closely spaced metal electrodes were deposited onto the 

surface of the crystal, and the structure was formed by the 

application of a voltage. After forming, the device showed a 

diode characteristic, and emitted intense blue light at a forward 

bias in the range of 10 - 40 V. Under pulsed conditions, the 

spectrum showed strong similarities to the lasing spectrum 

obtained from the same sample under cathodoluminescence. 

this device was not sufficiently stable for practical use, it 

Although 

appears to have demonstrated hot-carrier-induced 

CdS . diode lasing in 

53 
There are several lasing mechanisms in CdS, as evidenced by 

the varieties of emission spectra for different doping levels and 

temperatures. For lightly doped material, electron beam pumped 

laser emission at room temperature is thought to be due to free 

exciton recombination, whereas in doped material it may be due to 

free electron to acceptor recombination 142 . A study of degradation processes in electron beam pumped CdS 

102 has already been mentioned . 

SECTION X. 

CONCLUSIONS 

It may be asked whether there is any one approach which is 

more promising than the others, in the sense that a reasonable 

amount of development is likely to lead to well performing 

devices. As it is hard to avoid subjective factors from entering 

such an assessment, it is only possible to offer the following 

comments. Certain approaches can be criticized on the grounds 

that they do not provide the advantages of diode lasers, namely 

small size and high efficiency. This would remove from the top 

contenders frequency multiplication and summing, and electron 

beam pumped lasers. Non-p-n junction devices are faulted because 

of their reduced injection efficiency, although this may eventually be overcome by developments in both laser structures and 

injection methods. Other approaches can be thought of as 

requiring a considerable amount of development. Thus, the invention of new materials appears to require a long-term program, with 

a number of uncertainties. The same observations can be made 

54 
about the development of unmatched heterojunctions, and about the 

effort required to develop direct bandgap materials using new 

superlattice structures or interstitial doping. Perhaps the most 

promising short-term approaches are the modification of existing 

111-V materials through the addition of nitrogen as in the growth 

of AlGaAsN, and a renewed attack on certain well known materials, 

for example GaN and ZnSe, using novel growth techniques, perhaps 

microwave excited MOCVD or reactive MBE. 

SECTION XI. 

ACKNOWLEDGMENTS 

The assistance and cooperation of NASA Langley Research 

Center Library personnel has been invaluable. Appreciation is 

also expressed to Jean Peters Do for help with the preparation 

of the manuscript, and to Terry Mack and Anthony Cook for 

their cooperation. Finally, this project would not have been 

possible without the encouragement, help and assistance of 

Herbert D. Hendricks of NASA Langley Research Center. 

55 
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68 
P-doped cladding 

region 

Egl> Eg2 nl< n2 

Active region 

N-doped cladding 

region 

Eg3>Eg2 n3cn2 

Fig.1 Basic structure of a Diode Laser. 

69 
Fig.2 Power curve of a typical InGaAsP Laser. 

70 
2.5 

2.0 

1.5 

0 .5 1 .o 

GaAs X AlAs 

1.96 

0.38 

Fig.3 Direct and Indirect Bandgaps in AlGaAs. 

71 
Structure ................................................................ 

1 Depletion region 

v / 

flP Nq / / 

v / 

flP Nq / / 

; a) no bias ................................................................ 

---------------- 

Fermi level, EF i 

P/ 

/P N 9- / 

/ / 

I t--------J + 'Y 

b)forward bias 

Band diagram .......................................................................... 

q'built in 

---------------- *

'-\ 

Fermi level, EF 

.......................................................................... 

q(vbuilt in - vf) 

I 

Li 

Fig.4 Graded-Index Separate-Confinement Quantum-Well structure. 

72 
...... .. 

: Refractive i 

Structure i . Banddiagram i index 

T peof 

Lser ........................ ................................................................................................... 

Standard 

Graded index 

separate 

confinement 

I//////In Active 

\ 

Active, 1 

quantum well 

n 

n 

n 

n 

Fig.5 Structure and Band-Diagrams of a p-n Junction. 

73 
- Electron injection 

* L Hole injection a) symmetrical ............................................................................................................................. 

Electron injection 

i b) Unsymmetrical _____) 

Hole injection 

Fig.6 Electron and Hole Injection in a p-n Junction. 

74 
Barrier = metal work function 

- electron affinity 

Semiconductor electron affinity Vacuum level 

Metal work function 7 work function I 

EF I 

(a) No bias 

h - 

EF 

EF Metal N-type semiconductor . (b) Forward bias qvf 

Fig.7 Metal-Semiconductor Barrier, a) without and b) with 

forward bias. 

75 
No bias 

----------- 

I 

__---------- 

L 

Forward bias 

r W 

Fig.8 Mechanism of the Tunneling MIS diode. 

76 
n- AI GaAs 

Fig.9 AlGaAs/GaAs Heterointerface, a) without and b) with 

forward bias. 

77 
Selective 

Diode Nd:YAG mirror 

Mg0:LiNbOs 

I 1 I r 

\1047nrn 523.6 nm 

\ 

Optical cavity 

Fig.10 Structure of a Frequency-Conversion Laser. 

78 
n=2 

n=l 

- 

n=2 

n=l 

Fig.11 First two energy levels in the a) infinite, 

and b) finite Potential Well. 

79 
Fig.12 Density of States function in the conventional 

(3 dimensional) structure, and in a single 

Quantum Well (2 dimensional) structure. 

80 
a) no bias 

............. ............. r'Ilfi -------- BiasEF 

EF ______- 

3. 

b) "resonant" bias applied 

Fig.13 In Resonant Tunneling, application of the correct 

bias lines up the Fermi level with the first 

allowed level in the Quantum Well. 

81 
Report Documentation Page 

1. Report No. 

NASA CR-4252 

2. Government Accession No. 

I 

4. Title and Subtitle 

9 Security Classif (of this report) 

Unclassified 

Approaches Toward a Blue Semiconductor Laser 

20 Security Classif (of this pagel 21 No of pages 22 Price 

Unclassified 92 A0 5 

7 Authorls) 

I. Ladany 

9. Performing Organization Name and Address 

Planning Research Corporation 

Hampton, VA 23666 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Langley Research Center 

Hampton, VA 23665-5225 

3. Recipient's Catalog No 

5. Report Date 

September 1989 

6. Performing Organization Code 

8. Performing organization Report No 

10. Work Unit No. 

506-44-21-01 

11. Contract or Grant No. 

NASl-18000 

13. Type of Report and Period Covered 

Contractor Report 

9 lR7 - RR 

14. Spokoring Ag&y Code __ 

5. Supplementary Notes 

Langley Technical Monitor: Herbert D. Hendricks 

6. Abstract 

'ossible approaches for obtaining semiconductor diode laser action in the blue 

.egion of the spectrum are surveyed. 

I review of the current status of "visible" emitters, presently li'mited to 670 nm. 

lethods are discussed for shifting laser emission toward shorter wavelengths, 

ncluding the use of 11-VI materials, the increase in the bandgap of 111-V materials 

iy additions of nitrogen, and changing the bandstructure from indirect to direct by 

ncorporating interstitial atoms or by constructing superlattices. Non-pn-junction 

njection methods are surveyed, including avalance breakdown, Langmui r-Blodgett 

liodes, hetero-structures, carrier accumulation , and Berglund diodes. Prospects of 

nventing new multinary semiconducting materials are discussed, and a number of 

iovel materials described in the literature are tabulated. New approaches available 

:hrough the development of quantum wells and superlattices are described, including 

'esonant tunneling and the synthesis of arbitrary bandgap materials through multiple 

luantum wells. Also included is a review of the status of some well-known materials 

if interest for semiconductor lasers or emitters (Sic, ZnO, ZnS, ZnSe, GaN and InN, 

)i amond , CdS ) . 

Included is a discussion of diode lasers, and 

7. Key Words (Suggested by Author(sl) 1 18. Distribution Statement 

Jaser, Semiconductor, Materials, 

3andgap, Injection, Short-Wave1 engt h Unclassified - Unlimited