Quantum Dot Laser Seminar Report 2004

TABLE OF CONTENTS

INTRODUCTION 1

LASER 2

• PRICIPLE OF OPERATION 3 • LASER DIODE 4

™ PRINCIPLE OF OPERATION 5 ™ TYPES OF LASER DIODE 6

SEMICONDUCTOR LASER 7

QUANTUM WELL LASER 7

QUANTUM DOT LASER 8

ƒ QUANTUM DOT 9 ƒ GROWTH TECHNIQUES 12 ƒ CHALLENGES 13 ƒ SPECTRAL ANALYSIS OF QUANTUM DOT 14

LASERS

ƒ HIGH TEMPERATURE PROPERTIES 21

CONCLUSION 24

Dept of Electronics & Communication GEC Thrissur Quantum Dot Laser Seminar Report 2004

INTRODUCTION

Since the invention of semiconductor lasers in 1962, significant progress has been made in terms of high performance in many applications including telecommunications, optical storage, and instrumentation. Most modern semiconductor lasers operate based on quantum mechanical effects. Quantum well lasers have been used with impressive performance, while novel quantum dot lasers, a subject of intense research, show a great promise

Lasers come in many sizes and can be made from a variety of resonant cavities and active laser materials. Generally, increasing confinement enforces an increasing quantization in the energy of electrons. Therefore quantum dots, essentially zero-dimensional bits of material, will (once excited) re-emit light at nearly a single wavelength. Quantum dots are therefore a good starting point for producing laser light Some existing quantum dot lasers employ dots made epitaxially: the atoms in the dots are laid down meticulously using beams of atoms or molecules In the MIT laser the gain medium consists of nm-sized particles of CdSe coated with a layer of organic molecules and then immersed in a glassy film. The medium sits in a waveguide atop a grating. The fabrication advantage in this case derives from the fact that one uses simple solution processing rather than the more exacting technique of epitaxy usually needed for semiconductors. Furthermore, the color of the output laser light can be varied by changing the size of the CdSe particles, the grating spacing, or the refractive index of the waveguide, giving great flexibility to the design and application of the laser

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LASER

A Laser (light amplification by stimulated emission of radiation) is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light. Light from a laser is often very collimated and monochromatic, but this is not true of all laser types.

Common light sources, such as the electric light bulb, emit photons in all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent; i.e., there is no fixed phase relationship between the photons emitted by the light source.

By contrast, a laser generally emits photons in a narrow, well-defined beam of light. The light is often near-monochromatic, consisting of a single wavelength or color, is highly coherent and is often polarised. Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds).

Laser light can be highly intense — able to cut steel and other metals. The beam emitted by a laser often has a very small divergence (highly collimated). A perfectly collimated beam cannot be created, due to the effect of diffraction, but a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some lasers, especially semiconductor lasers due to their small size, produce very

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divergent beams. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources can generally not be collimated.

A laser medium can also function as an optical amplifier when seeded with light from another source. The amplified signal can be very similar to the input signal in terms wavelength, phase and polarisation; this is particularly important in optical communications.

The output of a laser may be a continuous, constant-amplitude output (known as c.w. or continuous wave), or pulsed, by using the techniques of Q-switching, modelocking or Gain-switching. In pulsed operation, much higher peak powers can be achieved.

Principle Of Operation

The basic physics of lasers centres around the idea of producing a population inversion in a laser medium by 'pumping' the medium; i.e., by supplying energy in the form of light or electricity, for example. The medium may then amplify light by the process of stimulated emission. If the light is circulating through the medium by means of a cavity resonator, and the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. Eventually it will get so strong that the gain is saturated (reduced). In continuous operation, the intracavity laser power finds an equilibrium value which is saturating the gain exactly to the level of the cavity losses. If the pump power is chosen too small (below the 'laser threshold'), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers.

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A great deal of quantum mechanics and thermodynamics theory can be applied to laser action (see laser science), though in fact many laser types were discovered by trial and error.

Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes in 1953. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was then known. The word laser was coined in 1957 by Gordon Gould, who was also credited with lucrative patent rights in the 1970s, following a protracted legal battle

Even low-power lasers with only a few milliwatts of output power can be hazardous to a person's eyesight. The coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and permanent damage in seconds. Lasers are classified by wavelength and maximum output power into safety classes numbered I (inherently safe) to IV (even scattered light can cause eye and/or skin damage). Laser products available for consumers, such as CD players and laser pointers are usually in class I or II

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LASER DIODE

A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in a light-emitting diode. Laser diodes are sometimes referred to (somewhat redundantly) as injection laser diodes or by the acronyms LD or ILD.

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Principle of operation

When a diode is forward biased, holes from the p-region are injected into the n-region, and electrons from the n-region are injected into the p- region. If electrons and holes are present in the same region, they may radiatively recombine—that is, the electron "falls into" the hole and emits a photon with the energy of the bandgap. This is called spontaneous emission, and is the main source of light in a light-emitting diode.

Under suitable conditions, the electron and the hole may coexist in the same area for quite some time (on the order of microseconds) before they recombine. If a photon of exactly the right frequency happens along within this time period, recombination may be stimulated by the photon. This causes another photon of the same frequency to be emitted, with exactly the same direction, polarization and phase as the first photon.

In a laser diode, the semiconductor crystal is fashioned into a shape somewhat like a piece of paper—very thin in one direction and rectangular in the other two. The top of the crystal is n-doped, and the bottom is p- doped, resulting in a large, flat p-n junction. The two ends of the crystal are cleaved so as to form perfectly smooth, parallel edges; two reflective parallel edges are called a Fabry-Perot cavity. Photons emitted in precisely the right direction will be reflected several times from each end face before they are emitted. Each time they pass through the cavity, the light is amplified by stimulated emission. Hence, if there is more amplification than loss, the diode begins to "lase".

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Types of laser diode

The type of laser diode just described is called a homojunction laser diode, for reasons which should soon become clear. Unfortunately, they are extremely inefficient. They require so much power that they can only be operated in short "pulses;" otherwise the semiconductor would melt. Although historically important and easy to explain, such devices are not practical.

Double heterostructure lasers

In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is GaAs with AlGaAs. Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article is referred to as a "homojunction" laser, for contrast with these more popular devices. The advantage of a DH laser is that the region where free electrons and holes exist simultaneously—the "active" region—is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification—not so many are left out in the poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the light is confined to the region where the amplification takes place.

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SEMICONDUCTOR LASER

Semiconductor lasers are key components in a host of widely used technological products, including compact disk players and laser printers, and they will play critical roles in optical communication schemes. The basis of laser operation depends on the creation of nonequilibrium populations of electrons and holes, and coupling of electrons and holes to an optical field, which will stimulate radiative emission. Calculations carried out in the early 1970s by C. Henry (Dingle and Henry 1976) predicted the advantages of using quantum wells as the active layer in such lasers: the carrier confinement and nature of the electronic density of states should result in more efficient devices operating at lower threshold currents than lasers with "bulk" active layers.

QUANTUM WELL LASER

In addition, the use of a quantum well, with discrete transition energy levels dependent on the quantum well dimensions (thickness), provides a means of "tuning" the resulting wavelength of the material. The critical feature size-in this case, the thickness of the quantum well-depends on the desired spacing between energy levels. For energy levels of greater than a few tens of millielectron volts (meV, to be compared with room temperature thermal energy of 25 meV), the critical dimension is approximately a few hundred angstroms. Although the first quantum well laser, demonstrated in 1975, was many times less efficient than a conventional laser (van der Ziel et al. 1975), the situation was reversed by 1981 through the use of new materials growth capabilities (molecular beam epitaxy), and optimization of the heterostructure laser design (Tsang 1982).

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If the middle layer is made thin enough, it starts acting like a quantum well. This means that in the vertical direction, electron energy is quantised. The difference between quantum well energy levels can be used for the laser action instead of the bandgap. This is very useful since the wavelength of light emitted can be tuned simply by altering the thickness of the layer. The efficiency of a quantum well laser is greater than that of a bulk laser due to a tailoring of the distrubution of electrons and holes that are involved in the stimulated emission (light producing) process.

The problem with these devices is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower refractive index than the centre layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.

QUANTUM DOT LASER

Even greater benefits have been predicted for lasers with quantum dot active layers. Arakawa and Sakaki (1982) predicted in the early 1980s that quantum dot lasers should exhibit performance that is less temperature- dependent than existing semiconductor lasers, and that will in particular not degrade at elevated temperatures. Other benefits of quantum dot active layers include further reduction in threshold currents and an increase in differential gain-that is, more efficient laser operation (Asada et al. 1986).

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Quantum dots.

A quantum dot is a potential well that confines electrons in three dimensions to a region of the order of the electrons' de Broglie wavelength in size, a few nanometers in a semiconductor. Compare to quantum wires and quantum wells.

Because of the confinement, electrons in the quantum dot have quantized, discrete energy levels, much like an atom. For this reason, quantum dots are sometimes called "artificial atoms." The energy levels can be controlled by changing the size and shape of the quantum dot, and the depth of the potential.

A potential well is the region surrounding a local energy minimum.If potential energy is imagined as corresponding to the height of the Earth's surface on a map, so that the resulting landscape of hills and valleys is a potential energy surface, then a potential well would be the region around a minimum of potential that could be filled with water without any flowing away toward another minimum

Quantum dots are so small that quantum mechanical effects come into play in controlling their behavior. Quantum mechanics apply in the microscopic realm but its effects are largely unseen and unfelt in our macroscopic world

Stimulated recombination of electron-hole pairs takes place in the GaAs quantum well region, where the confinement of carriers and of the optical mode enhance the interaction between carriers and radiation In particular, note the change in the electronic density of states, as a function of

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the "dimensionality" of the active layerThe population inversion (creation of electrons and holes) necessary for lasing occurs more efficiently as the active layer material is scaled down from bulk (3-dimensional) to quantum dots (0-dimensional). However, the advantages in operation depend not only on the absolute size of the nanostructures in the active region, but also on the uniformity of size. A broad distribution of sizes "smears" the density of states, producing behavior similar to that of bulk material

With the demonstration of the high optical efficiency self-assembled formation of quantum dots formed without need of external processing and having the natural overgrowth of cladding material (which addressed issues of surface recombination), there ensued a marked increase in quantum dot laser research.

The first demonstration of a quantum dot laser with high threshold density was reported by Ledentsov and colleagues in 1994. Bimberg et al. (1996) achieved improved operation by increasing the density of the quantum dot structures, stacking successive, strain-aligned rows of quantum dots and therefore achieving vertical as well as lateral coupling of the quantum dots. In addition to utilizing their quantum size effects in edge- emitting lasers, self-assembled quantum dots have also been incorporated within vertical cavity surface-emitting lasers. Table 5.4 gives a partial summary of the work and achievements in quantum dot lasers.

As with the demonstration of the advantages of the quantum well laser that preceded it, the full promise of the quantum dot laser must await advances in the understanding of the materials growth and optimization of the laser structure. Although the self-assembled dots have provided an

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enormous stimulus to work in this field, there remain a number of critical issues involving their growth and formation: greater uniformity of size, controllable achievement of higher quantum dot density, and closer dot-to- dot interaction range will further improve laser performance.

Better understanding of carrier confinement dynamics and capture times, and better evaluation of loss mechanisms, will further improve device characteristics. It should be noted that the spatial localization of carriers brought about by the quantum dot confinement may play a role in the "anomalous" optical efficiency of the GaN-based materials, which is exceptional in light of the high concentration of threading dislocations (~ 108 - 1010 cm-2) that currently plague this material system. The localization imposed by the perhaps natural nanostructure of the GaN materials may make the dislocation largely irrelevant to the purely optical (but not to the electrical) behavior of the material.

Quantum dot lasers work like other semiconductor lasers, such as those found in home-audio compact disc players. Just as in the semiconductor laser chip in a CD player, the goal of a quantum dot laser is to manipulate the material into a high energy state and then properly convert it to a low energy state. The result is the net release of energy, which emerges as a photon.

. In quantum dots, the electrons are confined within a very small volume that forces them to strongly interact with each other. These strong interactions can lead to deactivation of the dot through the so-called "Auger process," preventing it from emitting a photon.

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Quantum dots offer this performance over a range of temperatures, making them suitable for a variety of applications, and also can be "tuned" to emit at different wavelengths, or colors. The emission wavelength of a quantum dot is a function of its size, so by making dots of different sizes scientists can create light of different colors

GROWTH TECHNIQUES

Several growth approached have been developed to fabricate Quantum Dots arrays with high luminescence efficiency and low dislocation density using Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD). The parameters of such QDs arrays (QD density, size and shape) can be controlled by growth conditions. The emission range of InAs-GaAs nanostructures is extended up to 1.75 m at room temperature

Parameters of some QD lasers.

Wavelength Output Growth approach Growth power, technique 300K

1.3 m m 2.7 W Activated Alloy Phase MBE Separation

1.1 m m 3.7 W Stacking of QDs MOCVD

0.94 m m 4 W Submonolayer deposition MBE

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QD Lasers grown by MBE Energy diagram of the layers in a QD “dots-in-a-well” laser.

CHALLENGES

Thus, the challenge in realizing quantum dot lasers with operation superior to that shown by quantum well lasers is that of forming high quality, uniform quantum dots in the active layer. Initially, the most widely followed approach to forming quantum dots was through electron beam lithography of suitably small featured patterns (~300 Å) and subsequent dry- etch transfer of dots into the substrate material. The problem that plagued these quantum dot arrays was their exceedingly low optical efficiency: high surface-to-volume ratios of these nanostructures and associated high surface

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recombination rates, together with damage introduced during the fabrication itself, precluded the successful formation of a quantum dot laser.

The challenge, however, is that there are competing mechanisms by which the energy can be released, such as vibrational energy or electron kinetic energy

SPECTRAL ANALYSIS OF QUANTUM DOT LASERS

Since the early eighties, predictions have indicated that quantum-dot lasers should have superior characteristics to other higher dimensional structures such as quantum well devices and, with the advent of the self- organized growth technique, progress towards this goal has been made—at the present time, the best results being for lasers incorporating InGaAs or InAs dots. One unexpected feature of InGaAs/GaAs quantum-dot lasers is the nature of the longitudinal mode distribution. It has been observed that the laser emission spectra are broad and consist of peaks at regularly spaced intervals (approx 1–5 nm) superimposed on the normal longitudinal Fabry– Perot modes. Such behavior has been attributed to the discrete nature of the dots and the resulting inhomogeneous broadening (lack of a global Fermi function) leading to either spatial or spectral hole burning. Further hypotheses have been advanced to account for the periodic nature of the spectra where different subsets of dot sizes contribute to different groups of modes, the groups of longitudinal modes do not necessarily have a regular spacing . The suggested mechanisms include intracavity photon scattering a nonuniform distribution of dot electronic

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states (due perhaps to some preferred dot sizes), a gain that is dot size or shape dependent (due to size and shape dependence of either the oscillator strength or the efficiency with which dots capture carriers) and a modulation of the losses by constructive interference with the reflection of a transverse leaky mode propagating in the transparent substrate. The effects due to the leaky mode have previously been reported in quantum well lasers operating at the same wavelength. They lead to an optical mode loss and an optical confinement factor that vary as a function of wavelength with a period that is inversely proportional to the device thickness. The laser structure we have examined is represented in Fig. 1 and consists of three layers of InGaAs quantum dots each of which is grown in a

matrix of GaAs (10 nm thick). These are themselves grown in Al0.3Ga0.7As, and together comprise the waveguide core of the device. Atomic force microscopy (AFM) studies indicate the dots are lens like in shape, are 2.2 nm high and 36 nm in diameter with a dot density of 4.5x1010 cm -2.

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FIG. 3. Spectra of the device of Fig. 2 taken at 1.4 and 1.53Ith and at a temperature of 280 K. The larger wavelength range shows the presence of a second group of lasing modes at higher energies. The two spectra are offset on the vertical scale.

Typical spectra for 50 mm wide oxide isolated stripe devices fabricated from the above structure are presented in Fig. 2 (a)The spectra were measured, using a spectrum analyzer(0.07 nm resolution), as a function of drive current (I=1.1, 1.2, 1.3, and 1.4x Ith ) at a temperature of 280 K. The devices being operated pulsed with a pulse length of 300 ns and a duty cycle of 0.03%. In addition to the normal longitudinal modes (spacing ~0.09 nm for the device that is 1500 mm long), which we can just resolve with the spectrum analyzer and just pick out in the spectrum shown magnified in the inset, there is a more widely spaced periodicity present in the data. The groups of longitudinal modes or supermodes are much more obvious than the longitudinal modes themselves and have a spacing of approximately 0.6 nm.

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The Fourier transform of each of the four spectra (plotted in terms of wave number so that the conjugate variable is length) are shown in Fig. 2(b) and demonstrate the presence of a periodicity in all four spectra even at the relatively low drive currents used here. At still higher currents, lasing spreads to a second group of higher lying energy states as shown in Fig. 3 for spectra recorded at currents of 1.4 and 1.5x3Ith. This second group of modes complicates the Fourier transform, introducing extra detail, but the Fourier transforms of each of the two groups taken individually indicate a similar periodicity within each group.

FIG. 2.a Quantum-dot laser spectra taken at drive currents of 1.1, 1.2, 1.3, and 1.43Ith and a temperature of 280 K for a 50 mm wide, 1500 mm long oxide isolated stripe device. The spectra have been offset on the vertical scale for ease of comparison ~higher current have larger offsets!. The spectra exhibit groups of longitudinal modes separated by approximately 1nm intervals in addition to the normal longitudinal modes shown in the magnified section of the 1.33Ith spectrum in the inset.

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Fig 2.b. Fourier transforms of the data in (a) plotted in terms of wave number. The spectra are offset on the vertical scale for clarity (increasing offsets for higher currents).

The spectra of the three sets of quantum-dot lasers with different substrate thickness were measured as a function of drive current and temperature. Fourier transforms were used to simplify the analysis of the spectra. As recently shown by plotting the spectra in terms of wave number, the Fourier transform gives information about the optical path length within the laser cavity. Furthermore, by using the refractive index and refractive index energy dependence this information can be converted into the device

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length. The length dependence of any other periodicity within the spectrum also then becomes apparent. In Fig. 4, we have plotted the Fourier transform of the wave-number spectrum of a 1500 mm long, 260 mm thick device operated at 23Ith and at a temperature of 150 K. The low temperature allows us to drive the device well above threshold without exciting the higher energy states observed in Fig. 3. In the upper trace, which is the lower trace amplified by a factor of 20, a feature exists at both the device length (B1) and twice the device length (B2). The largest feature (A1), which is readily apparent in the trace that has not been amplified, corresponds to a length of250 mm, with another feature (A2), apparent in the amplified trace, at 500 mm. Similar measurements taken on the other devices of different thickness and cavity length are summarized in Table I. The features apparent in the Fourier transform spectra, which represent the periodicity present in the measured spectra, show a correlation with the thickness These results indicate that the dominant mechanism leading to the regular modulation of the emission spectra in these quantum-dot lasers is related to the device thickness, although there are some additional features present in some of the measured spectra that do not appear to be related to the cavity length or thickness. It may be that in quantum-dot devices where substrate effects are suppressed that other mechanisms cause regular or quasiregular mode distributions.

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FIG. 3. Spectra of the device of Fig. 2 taken at 1.4 and 1.53Ith and at a temperature of 280 K. The larger wavelength range shows the presence of a second group of lasing modes at higher energies. The two spectra are offset on the vertical scale.

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HIGH TEMPERATURE PROPERTIES

The growth of selforganized InAs quantum dots allows the realization of lasers emitting at 1,3 µm on GaAs substrates. Beside the principal advantage that GaAs substrates can be used instead of InP substrates for large volume production quantum dot related effects like very low transparency current densities and low internal absorption are also of importance. Other predictions of quantum dot lasers like low temperature dependence could not be realized up to now for room temperature and above. Especially in the case of 1.3 µm emitting quantum dot lasers good high temperature performance is still a problem. We have realized 1,3 µm emitting graded index separate confinement heterostructure lasers with InAs dots embedded in a 10 nm thick Ga0.85In0.15As quantum well. Our structure additionally uses short period superlattices (SSLs) in the graded regions to improve the carrier confinement by electron back reflection [1, 2]. This improvement allow ground state lasing at temperatures > 80 °C. Multi quantum dot structures with large dot layer separation of 50 nm were used to avoid any strain coupling and to minimize strain accumulation. The growth temperature for the quantum dot layers was 510 °C and for the 1.6 µm thick cladding layers

570 °C, respectively. The influence of the amount of quantum dots on the laser performance was investigated by varying the number of dot layers from 3 to 8 layers. The best results were obtained with 6 uncoupled quantum dot layers with transparency current densities of less than 40 A/cm2 ( Fig. 1), an internal quantum efficiency of about 35% and an internal absorption of 1-2 cm-1. Ridge waveguide lasers with 4 µm ridge width and cavity lengths as short as

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800 µm long can be operated at room temperature in cw mode without any facet coatings. These devices show good temperature characteristics with T0 > 70 K up to about 50 °C and 54 K up to 140 °C, respectively ( Fig. 2). The maximum operation temperature was above 150 °C which is the highest value known up to now for 1.3 µm emitting quantum dot lasers.

Fig. 1: Threshold current density of 2 samples with different numbers of quantum dot layers as function of the inverse cavity length. Values determined in pulsed operation for 100 µm wide broad area lasers at 20 °C. Due to the improved gain by 6 dot layers with an average dot density per layer of about 1x1011 cm-2 and the low internal absorption high performance short cavity devices could be realized using high reflection facet coatings (83% for front and 95% for backside facets, respectively). 400 µm long devices exhibit threshold currents as low as 6 mA and more than 5 mW output power at 30 mA Emission from the fundamental dot states was

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achieved from cw operating unmounted devices up to 70 °C with more than 2 mW output power. The maximum cw operation temperature was 90 °C

Fig. 2: Temperature dependence of the threshold current density of a 2.5 mm long uncoated ridge waveguide laser with 6 quantum dot layers.

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CONCLUSION Though QD lasers show immense potential for superior device performances, there are still some significant problems associated with the control of emission wavelengths reproducibilty of the dots,high temperature reliability and long term stablity of the dots.The current challenge is to match and surpass the performance of the quantum well lasers.There is still need for the development of QD strength of lasing around 1.55 micrometre ,which is a principal wavelength in fibre optic communications.This would give QD lasers a chance to move into application such as ultrafast optical data transfer .A key aspect of QD production challenge will be to improve our control over the dot distribution produced in the self assembly process .Reliable continuous wave room temperature operation of QD lasers has already been reported; structure improvements are required to get the operation characteristics more desirable,especially the elimination of several mechanisms that have a detrimental effect at room temperature. From a bird’s eyeview ,the research on QD lasers is still newly emerging from its beginning stages .Several promient group of researchers around the world are all going down their own avenues ,grappling with a portion of the overall problem ,identifying and overcoming obstacles one by one individually .This is not surprising ,considering the research on QD lasers , as opposed to somewhat more well established research on basic QD’s themselves began to hit the stage truly only around 1995-1996.Still consideing the efforts and the emergence of well defined directions ,there seems to be hope that the field will settle down and become established .If the collective effort succeeds in bettering the performance of quantum well lasers ,which it might ,then QD lasers can finally be up there along with the MOSFET,quantum well lasers and monolithic integration technology.

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REFERENCES ƒ Long wavelength quantum dot lasers in Journal of materials science: Materials in electronics January 2002 ƒ 1.3 micro metre QD lasers with improved high temperature properties by F.Klopfs and R.Krebs ƒ Spectral analysis of InGaAs/GaAs quantum dot lasers by ƒ P.M.Smowton in Journal of Applied physics letters Volume 75,October 1999 ƒ www.wikipedia.org ƒ

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