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Quantum Dot Lasers
EE453
By: A. Kyle Donohue
Email:
Fall 2008


Introduction

The remarkable progress in the making of low-dimensional semiconductor structures throughout the last few decades has made it possible to reduce the effective device dimension from three-dimensional bulk materials, to quasi-two dimensional quantum well systems, to quasi-one dimensional quantum wires, and even to quasi-zero dimensional quantum dots. The modified electronic and optical properties of these quantum-confined structures, which are controllable to a certain degree through the flexibility in the structure design, have attracted considerable attention, and have made them very promising candidates for possible device applications in semiconductor lasers, microelectronics, non-linear optics, and many other fields.

Quantum dot lasers are revolutionary lasers that are significantly superior to conventional semiconductor lasers, in that they feature higher performance in such aspects as temperature-independent operation, low power consumption, long-distance transmission, and fast speeds. It is anticipated that quantum dot lasers will become a core technology to realize high-performance light sources for optical telecommunication, for which data traffic is continuing to increase rapidly [1]. It is in this report that I am going to be discussing a specific quantum dot device, which is known as the semiconductor quantum-dot laser.

Understanding a few terms associated with quantum dot lasers

To fully understand what a quantum dot laser is we must first define what a quantum dot is. To do this I will give a brief explanation. A quantum dot is a particle having an approximate size of one nanometer which has the display properties of a semiconductor. A semiconductor is a solid material that possesses some amount of electrical conductivity. Silicon is one of the most popular materials used in creating a quantum dot.

The size of the quantum dot, one-billionth of a meter, can cause it to exhibit unusual properties that are not present in larger samples of a semiconductor material. These properties could have some benefits to humans including, but not limited to, energy and light production. Unlike some forms of nanotechnology, the quantum dot is not theoretical. It has been created in a real-world setting.

The key to the quantum dot is in the electrons. Electrons occupy one of two bands in a material’s crystal. By providing the proper stimuli, an electron, or perhaps more than one, can be encouraged to move from one band to the other. As it moves from one band to the other, it creates a hole, which is positively charged. Together, the hole and the electron are referred to as an exciton.

The electron and the hole in the exciton normally keep their distance from each other. This is called the Exciton Bohr Radius. However, if the crystal is reduced in size, it crowds this gap. Once that happens, it changes the crystal’s ability to absorb and emit energy. At this point, the quantum dot is created. Different colors can be obtained by reducing or increasing the size of the quantum dot [2].

With the little knowledge we just gain about quantum dots we can continue our look into what a quantum dot laser is. A quantum dot laser is a semiconductor laser that uses quantum dots as the active laser medium in its light emitting region. Due to the tight confinement of charge carriers in quantum dots, they exhibit an electronic structure similar to atoms. Lasers fabricated from such an active media exhibit device performance that is closer to gas lasers, and avoid some of the negative aspects of device performance associated with traditional semiconductor lasers based on bulk or quantum well active media. Improvements in modulation bandwidth, lasing threshold, relative intensity noise, line-width enhancement factor and temperature insensitivity have all been observed. The quantum dot active region may also be engineered to operate at different wavelengths by varying dot size and composition. This allows quantum dot lasers to be fabricated to operate at wavelengths previously not possible using semiconductor laser technology [3].

The early beginning of quantum dot lasers

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. In 1974, Raymond Dingle of Bell Laboratories demonstrated quantum confinement of charge carriers for the first time, and in 1979, Won-Tien Tsang, also of the Bell Labs, built the first semiconductor laser based on quantum confinement [7]. Since then, quantum well lasers have become the backbone of fiber optic communication systems. Quantum wire lasers were studied in the late eighties. However, the early works of Yasuhito Arakawa and later of Mashahiro Ashada at Tokyo University in 1986 have shifted the interest of the optoelectronic community almost directly from quantum well to quantum dot lasers in search of greater benefits 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).

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

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.

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 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.

Idea of operation

In conventional double heterostructure lasers, a thin (0.1-0.3mm) active region of lower band gap material (e.g. GaAs) bounded on either side by a larger band gap material (e.g. AlGaAs), acts as a trap for electrons and holes, thereby reducing the required threshold current density. If the thickness of the active layer is reduced to 50-100Å, the dimension become comparable with the de Broglie wavelength of the thermalized electrons, and the confined electrons and holes display quantum effects [4], [5], [6].

Since the movement of the charge carriers is restricted in all three dimensions in a quantum dot, the degeneracy of energy levels is largely lifted, and the density of states becomes extremely quantized, as found by solving the time-independent three-dimensional Schrodinger equation. The smaller the dimensions of the quantum dot; the larger the separation between adjacent energy levels.

For efficient lasing, it is desirable to have a large density of carriers in both electron and hole bands at energies close to the band-edge, so that population inversion (i.e. there should be more electrons in the excited state than in the ground state) becomes easier. Short population time of ground states in quantum dot lasers ensures a huge number of optical transitions per unit volume, most of them being

radiative recombination, rather than non-radiative, resulting in high internal efficiency.

Energy pumped into the system raises the charge carriers from one energy level to the next; none of it goes to random motion, because there are no other degrees of freedom. Thus, one expects any lasing from the dots to occur with high efficiency and at lower threshold current than in either quantum wells or quantum wires.

Advantages of QD-lasers

1. Quantum dot lasers should be able to emit light at wavelengths determined by the energy levels of the dots, rather than the band gap energy. Thus, they offer the possibility of improved device performance and increased flexibility to adjust the wavelength [7].

2. Quantum dot lasers have the maximum material gain and differential gain, at least 2-3 orders higher than quantum-well lasers [8].

3. The small active volume translates to multiple benefits, such as low power high frequency operation, large modulation bandwidth, small dynamic chirp, small linewidth enhancement factor, and low threshold current.

4. Quantum dot lasers also show superior temperature stability of the threshold current.

5. In addition, quantum dot lasers suppress the diffusion of non-equilibrium carriers, resulting in reduced leakage from the active region.

6. More novel structures such as distributed feedback lasers and single-dot VCSELs promise ultra-stable single mode operation.

MARKET DEMAND AND NEW TECHNOLOGY

Because of the approved advantages of Quantum Dots Lasers, such as low threshold current, enhanced differential gain, lower chirp/high spectral purity, independent of the threshold current on temperature and a decreased a factor, QDs Lasers were intensively researched all through the previous decade. They are suitable to be used in optical applications, microwave or millimeter wave transmission with optical fibers and other telecom and datacom networks. However, QD lasers were commonly regarded as only a theoretical topic which is almost impossible to be brought to the market. The early models were based on the assumptions:

1.  Only one confined electron level and hole level

2.  Infinite barriers

3.  Equilibrium carrier distribution

4.  Lattice matched heterostructures

The emerge of self-assembling growth technology which forms today the very basis of optoelectronic devices such as edge emitting lasers, which has great potential for the future applications, pushes quantum dot lasers to the boundary between theoretical field and commercial applications. Those updated QD based lasers employ fundamentally different models compared to the original models:

1.  Lots of electron levels and hole levels

2.  Finite barriers

3.  Non-equilibrium carrier distribution

4.  Strained heterostructures

The predictions of decreased _ factor and wavelength chirp have already been proved on real devices. In the lightwave applications, lasing in the 1.3um spectral range, using GaAs substrate, both surface and edge emitters have been commercially produced at 6 inch diameter.

Nevertheless, as can be expected, due to the challenges listed below, the way of fulfill the QD based lasers into commercial markets is not smooth.

1.  First, the lack of uniformity.

2.  Second, Quantum Dots density is insufficient.

3.  Third, the lack of good coupling between QD and QD.

Recently, a Tunnel Coupling Layer for Efficient Quantum Dot Lasers technology has been published as a Commercial Opportunity Announcement. In order to enhance transportation of electron-hole pairs among quantum dots, get more efficient quantum dot lasers and break the limitation of the older QD technologies, a solution of coupling the sheet of uniform and dense layer of quantum dots, via a thin barrier, to a quantum well (QW) layer. This technology has been proved in the visible red wavelength. InAlGaP was used as the coupling barrier layers and InGaP was used in quantum well layers [26].

D. Bimberg, G. Fiol, M. kuntz et al. has stated that GaAs-based QD lasers will be a good choice for light wave communication networks in terms of performance and expense.

Although difficulties were met on the way of realizing QD lasers, with those attractive properties, Quantum Dot laser is still predicted to maintain a hot research field in a few years. Some researchers are seeking some other ways to push their research toward [9].

FUTURE

The advantages of quantum dot based lasers compared to other conventional technologies have been realized for several years. Especially the free geometric parameters of quantum dot layers give probabilities to tailor the spectral gain profile applied to different types of QD lasers applications.

Nevertheless, due to the intrinsic limitation of technologies, to realize quantum dot lasers with predicted properties met several difficulties. The requirement of further widening the parameters range in order to reducing the inhomogeneous linewidth broadening (we need homogeneous linewidth) is one of the aspects of developing quantum dot lasers. Using surface preparation technologies, lots of groups are working on the issue of further controlling the position and dot size for the self-organized technology. Once the developed methods can be implemented in the high density systems, the new technology will become the breakthrough in the history of quantum dot lasers development.

Since the speed of carrier capture extremely increase the transport time and affects the modulation bandwidth, it is required to decouple the carrier capture from the escape procedure.

Employing tunnel injections to quantum dots is a choice. Allowing the injection of cooled carriers, this method is able to achieve good performance without loosing the extra carriers which often happens before due to the thermal relaxation. With the experiment done by comparing the

QW lasers and QD lasers in term of raised gain at the fundamental transition energy with the constant broad band characteristics of quantum dot lasers, it is concluded the combination use of quantum dot and quantum well would tailor the material properties in a much wider range than using quantum dots or quantum wells alone.

With the employment of further control of parameters and better coupling technology and the breakthroughs which are already done, realizing quantum dot lasers as well as other quantum dot optoelectronic devices in commercial market is not so far away.